FallenWyvern
Villager
I hate to say this, but... I couldn't find the value-add of the tool. I downloaded it to see if it would auto-compute CR based on the DMG rules, and even better would be if it could auto-compute CR for a given spell list (currently I just guesstimate). But it doesn't seem to be designed for that purpose at all.
It's nothing you can't do just on paper (or in your editor of choice) but it's to format it and make it look professionalish. I tried to do CR guessing (there's some disabled code in there if I ever decide to chase it further) but the problem is while I can easily calculate CR for things like attacks and damage, it gets totally useless once you hit abilities and non-damaging abilities. "Multiattack: This creature gets two claw and one bite attack". That's a significant 'average damage' over the two types of attacks separately, and yet there's no real way to parse that intelligently. Spellcasting, as you noted, is just eyeballed from DM to DM and is an art, which computer programs aren't good at.
I'd say if I was to propose the 'value add' of such a tool was to rapidly create the output. There are redditors that I built this tool for, because they were doing this all in photoshop by hand or by editing an html file each day. This speeds up that process.