I’ve made my own soap from saved animal fat and lye (not from the clinic you sick bastards!)
There are sites out there that have calculation tables for the type of fat you’re using so you can slightly “overfat” your soap, to make sure you fully consume the lye during the process and end up with non-caustic soap.
Made a batch of beef tallow/coconut oil lavender soap for my wife last christmas. Turned out pretty well if I do say so myself.
What percentage to potassium hydroxide to overall liquid?
If you’re using the soap to wash dishes, then having a bit of excess lye is fine, because it will react with the fat on your pans to make yet more soap.
Given the chemicals involved, I think I’d prefer to use home made soap on things like dishes, rather than my face.
You beat me to it. LOL 
The ratios of potassium or sodium hydroxide to oil can vary considerably depending on what oils you’re using - the molar masses of oils can be very different. A mass ratio of lye to coconut oil, used unaltered with a mix of canola and corn oil, would leave a tremendous amount of unreacted lye in with the soap - it would be very caustic and unfit to use on skin.
There are quite good soap calculators, I’d use one of them.
I worked in a huge cafeteria kitchen for a little while - their big dishwasher, when running at full speed, took about ten people to operate. Mostly I was handwashing the cooking dishes, but at the end of the lunch rush I was pulled from that duty and worked on the disassembly line putting the eating dishes through the big dishwasher.
That machine used pure lye, which came in 20 kg or so bags, as its detergent. When someone accidentally punctured a bag of dish detergent in the storage room, the cleanup was a pretty involved operation. After sweeping up as much of it as we could, we mopped and mopped the area with vinegar. Probably used a couple of gallons of vinegar (they also went through so much white vinegar they didn’t bother buying it in containers - a tanker truck came every couple of weeks and filled up the tank by the loading dock)
Pedro, is that you? 
(I sometimes get very nice soap from a friend who does the same thing.)
Keeps the fat out of the drains, which helps control the local municipal budget.
You really shouldn’t have any lye left. But if you do, don’t use it on aluminum dishes.
Isn’t that just a dumpster dive behind the clinic?
It’s really not that hard if you’re paying attention and taking all the recommended precautions.
You have to really screw up to make caustic soap.
If you use a lye calculator, allow for a generous superfat percentage, and carefully weigh the lye out, your end product will be safe.
There can be a likelihood of having an undissolved lye clump in the soap, but that’s why the lye should be fully dissolved in water first.
Also, using a cheap immersion blender to mix the lye solution thoroughly further prevents lye spots from happening.
A bar can still be a little harsh if for whatever reason there’s still some unreacted lye after all the fat’s been saponified, but it’s not eat your face off bad, plus allowing it time to sit and cure will mellow it out by having it react with the carbon dioxide in the air to neutralize out into soda ash.
Some people use pH strips, but the tongue test works too.
Touch the tip of your tongue to a bar. If it tastes like soap, you’re good, but if you get a zap, it still needs time to cure.
That’s an excellent rundown of the caustic soap problem. I was referring to a less subtle issue: working with high molarity sodium hydroxide is inherently dangerous, because lye will eat your face. Drop the lye pitcher at the wrong time, lye splashes, chemical burns, ow ow ow. It’s a disproportionally risky chemical hazard to work with in pursuit of a product that isn’t worth very much.
One can approach melt and pour soap casually. Saponification, though–when we’re dealing with quantities large enough to end up with full-sized bars of soap–you have to be pretty careful with.
This is the saponification table I usually work with, from Majestic Mountain:
https://www.thesage.com/calcs/LyeCalc.html
It has enough of a variety of fats and oils that you can mix and match almost anything to make your soap. The hard part may be getting straight lye to process it; a lot of places have discontinued carrying pure lye because they assume we’re all going Walter White on them.
I got my lye from “The Lye Guy” (Jim Sweeting) in 2009. The same web site is still up, and appears in a few lists of lye suppliers of more recent vintage.
Oh, yeah, totally.
I guess I misread.
The lye is dangerous, and handling it can be dangerous, so precaution is definitely needed. I always wear gloves, goggles and an n95 mask, just in case.
But the end result, the actual soap, being dangerous, that can only happen if someone pretty much ignores the proper process almost entirely.
You can get 5 lbs of food grade lye on amazon for $20. I also use it to make pretzels and bagels.
Or, um, swimming in the Caribbean.
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