Hi Hywel and welcome to the forum
Thank you for your kind words above. To help people like yourself to become confident in their curing is one of the major reasons we set up the forum.
A few bits of basic background information and advice...
There are strict UK/EU/UDSA regulations on the use of cures in commercially produced bacon and, as you would expect, these have safety margins built in to them. Over the last few years we have seen the amount of permitted cure almost half - from ~270 mg/kg (ppm) to 150 mg/kg (ppm). Even though this is the case, the EU and UK still allow higher levels of cure in "traditional" cured meat products. On the forum here we only recommend the use of the standard commercial cure levels and so you can feel safe using the calculations you see.
Unfortunately there are no legal cure limits when producing home made bacon and many of the widely available cures (like Supracure), when used at the recommended rate, can result in cure levels being double the permitted commercial levels. Should you avoid these home cure mixes? No, not necessarily. Once you become comfortable calculating the cure you are using (which you should ALWAYS do - even if it just for your own peace of mind) you can often adapt their usage rate to bring them back into line. Supracure for instance, when used at 50% of their recommended rate (e.g. 2.5% instead of 5%) results in salt levels of 2.5% and Nitrite levels of 150 mg/kg - a good result all round.
The good news is that many of the newer cure mixes that are coming onto the home market do conform to the maximum commercial UK/EU cure limits.
If the cure that you are thinking of buying does not tell you exactly what is in it, first contact the supplier and ask them to tell you the contents in writing. If they cannot/will not tell you then don't buy it.
If you are making bacon then make sure that the cure you buy contains only NitrIte and does not contain any NitrAte as well. Of the two it is the Nitrite that is the active component and for relatively short shelf life products (like bacon) is the one that you need. The Nitrate is only required in the longer shelf life products (like salamis and other air dried sausages, or air dried meats like "Parma" ham, bresaola or pancetta). Here the Nitrate acts as a Nitrite reserve - over time it slowly breaks down to Nitrite to keep the Nitrite levels topped up. Using curing salts that include both Nitrite and Nitrite in something like bacon only means that you are using up to twice as much curing salt that you think you are.
Limiting the Nitrite/Nitrate in your bacon is not really about making the bacon "safe" but more about helping to reduce the overall amount of Nitrate/Nitrite that we eat in our overall diet. When you eat your bacon steak with a good helping of cabbage, carrots and maybe some braised celery, it is likely that each of the vegetable portions could contain as much (or maybe even more) Nitrate/Nitrite than the bacon itself.
The more accurately you can weigh the cure the better however we can sometimes get over paranoid over accuracy. We need to remember that meat is biological tissue and so each joint/belly will actually absorb different amounts of the cure. When calculating the levels we are measuring the amount of cure we are applying to the meat ("ingoing") and not the final final amount in it ("residual") and that you are eating. It all works on averages and so the residual amount in the end bacon will vary. Official testing over the years has shown that the use of the published maximum "ingoing" amounts will result in the "residual" cure being within the effective protective range.
For a 1 Kg slab of pork belly you would need 2.4 g of Cure #1 (6.25% Nitrite in Salt) to give you 150 mg/kg ingoing Nitrite. However if you only has scales that measured to the nearest gram then providing you weighed out more than 2 g but less than 3 g of the cure you would end up with Nitrite levels somewhere between 125-188 mg/kg - and these are well within the upper and lower safety margins. The closer you can get to weighing the 2.4 g of cure the better though.
Regarding the curing container... As we are carefully calculating the amounts of salt and cure (to provide a consistent quality of bacon) we need to ensure that the salt/cure/brine remains in close contact with the surfaces of the meat. The Pyrex dish approach would work for the techniques where we simply relying on an excess of salt for the curing process, however it is not recommended for techniques where we are using curing salts (e.g. Nitrite). This is due to the uncertainty of the amount of the cure that will be absorbed. To minimise the use of plastic you can use the ziplock food bags and remove as much air from the bag as you can while curing. After use the bags can then be washed out and re-used for your next batch. Not a complete avoidance of plastic but it will minimise its use over time. The use of bags like this keeps the fridge clean and allow you to cure multiple joints in stacks - one joint per bag.
The length of time you need to cure depends on the thickness of the meat. For most belly joints 6-7 days is usually sufficient however loin joints (back bacon) will usually require 10-14 days. Within reason you cannot "over cure" so if you leave either types of joint curing for 14 days (which is what I do) then you are fine. If you want to only cure the belly for 7 days then that is fine too.
The curing time is not dependant on the salt content. The calculated 5% or 2.5% salt is the estimated residual salt that has diffused into the bacon during the curing period. When you first apply the salt/cure to the meat the surface salt concentration will be 100% - this will kill most of the bacteria on the surface of the meat and give even the most resilient bacteria a very very bad day. At that point the pork will be fine throughout the curing period so long as it is kept refrigerated. After the cure the slab of cured bacon will then be fine for about 6 weeks so long as it is kept refrigerated. The salt does most of the preserving work however the Nitrite has some very important roles too - it gives bacon its "bacon" flavour, it inhibits the growth of botulinum and it prevents the meat fats from becoming rancid.
My advice is to keep the levels of salt in your bacon low. 2.5% is a level that suits most peoples tastes. This level still makes good lardons - and if they are not salty enough for your recipe then just adjust the seasoning accordingly. The two end slices of the bacon will be more salty than the inner slices so you could save these specifically for lardons.
Avoid any recipe that involves using curing salts that tells you to add more salt each day and pour away any brine. The results from this method will be so unpredictable it is, at best, a black art - and almost verges on alchemy. This method would be fine though if you are only using salt to cure, without the use of any curing salts.
I hope this helps to reassure you.