I recommend Shari's thread, because you'll learn a lot.
My answer to your specific question is this -- the amount of glycerin you use is completely up to you.
It is theoretically
possible to use no glycerin up to 100% glycerin, although I strongly do not recommend using 100% for safety reasons (see the indented paragraph below). Or any percentage in between. It's up to you.
I have made the same recipe with varying amounts of glycerin to see how the recipe behaves. They have all worked fine. See: https://www.soapmakingforum.com/thr...erin-water-vs-water-only-lye-solutions.51237/
A safer and easier maximum limit is to dissolve the dry KOH in water equal to the KOH weight. This is done at room temperature -- you do not heat this mixture. After the KOH is dissolved, add sufficient glycerin so the total weight of water + glycerin equals the total "water" weight called for in the recipe.
There's no way to give this amount as a fixed percentage. The percentage will vary depending on the superfat, fat blend, and lye concentration chosen by the soaper.
You can add the glycerin to the oils or to the water-KOH mixture -- there's no one right answer to this either. Do what seems best for you.
Pros of less glycerin: A bit more lather in the finished soap. Lower cost because water is cheap compared to glycerin.
Cons of less glycerin: May want to warm the starting soap batter to 160-170F / 70-75C to encourage the batter to come to trace faster. You do not have to cook the soap after it comes to trace, however -- you can still use a cold process method.
Pros of more glycerin: Comes to trace quicker at lower temperatures. Works well with a cold process method. Some claim the soap is thicker when diluted, although I don't see that.
Cons of more glycerin: Higher cost. Somewhat less lather.