Ditto what Dibbles said.....soap calculators are designed to calculate how much KOH or NaOH are needed to turn specific amounts of particular oils/fat into soap. Since glycerin is an alcohol and not a fat, it does not saponify, so there's no reason for calculators to include it in their saponification calculations.
Glycerin isn't a fat. You can add extra glycerin to soap, no problem, but it is misleading to think of it as superfat and it will only confuse others if you call it that.
Glycerin does not saponify, so it does not consume lye. It is a byproduct of saponification.
As far as how much glycerin is produced by the saponification reaction, here's an article of mine that explains how to calculate the amount -- https://classicbells.com/soap/soapystuff.asp If you have any questions about this article, please ask me.