Soap making isn't typically done using molarity/molality. We aren't doing quantitative analysis.
If you want to talk in terms of molar concentration, you'll get a lot of blank looks from soap makers.
We normally work in percentage by weight; that's sufficiently accurate for our purposes. The NaOH concentration used for soap making ranges anywhere from 25% to 50% w/w (meaning 25 g NaOH in 100 g finished solution, and so on) with a typical range being 28% to 33%.
The usual recommendation is to use an online
soap making calculator, unless you feel a great need to do calculations by hand. My suggestion is to check out
https://www.soapmakingfriend.com/soap-making-recipe-builder-lye-calculator/
Of course the calculations can be done by hand -- online calculators didn't exist 20 years ago -- but I don't think a lot of people do that nowadays given there are online calcs that are accurate and reliable. If you want to learn about the required calculations, however, here's a good article from a reputable soap maker about the process --
https://auntieclaras.com/2018/09/lye-calculation-tutorial/ If you have questions, I'll be happy to help.
There are several reasons why you see some variation in the answers from any soap calculation. One of the main reasons is that saponification values used for these calculations are averages. And the databases for the various calcs aren't all exactly the same because various sources for sap values report slightly different averages.
Unless you test YOUR fats for their sap value, the numbers you calculate won't be perfectly accurate either. The variability in sap values and alkali purity is an important reason for why most soap calculations include some superfat -- meaning a small amount of excess fat.
We provide an excess of fat as a safety factor to account for small errors in measuring out the ingredients and to allow for some inaccuracy in the numbers our recipes are based on. Based on my experience (chemical engineer and former chem lab tech), the margin required to ensure safety is 1-2% superfat. Anything higher than that is personal choice.
The chemistry you're dealing with is not a simple acid-base neutralization where the reaction endpoint is a black-and-white answer. The pH of finished soap
with no excess alkali will range from about 9 to about 11.5 depending on the fatty acids in the soap. A soap that has a pH of 10 might have excess alkalinity (not good) or it might have excess fat (desirable) -- the pH test alone simply cannot give you a definitive answer. There are other issues as well, but this is the key issue in my opinion about why pH alone isn't a particularly helpful test.
A titration for excess alkalinity is the correct test, so if you're wanting numbers to rely on, you will need the correct equipment, reagents, and procedure to do this titration. An alternative to an excess alkalinity test is what we call the "zap" test. You can find a tutorial about the zap test in the Lye Based Soap forum here.
I won't get into detail about the other issues you raise about not wanting to make a dangerous product. Suffice to say most of us, including me, can reliably and repeatedly make safe soap without ever testing excess alkalkinity, saponification values, purity analyses of the alkalis, etc. If you want to take the tack of measuring the numbers, by all means it can be done. But it's not strictly necessary to ensure product safety.
