I started working on a similar project about a year ago, except I was doing it fully by hand in the vanilla game (journey mode in a blank world), custom 8-bit instruction set, all that. I took an extended break from the project and kept thinking “this idea is so obvious, someone else is gonna do it first and I’m gonna look like a copycat” but not getting around to finishing work on it anyways. I’ll post pictures if anyone is interested, maybe a world download if I can find somewhere to host it.
I live in Washington state and I’m pretty certain the sales tax here is 10% (slightly higher than your maximum figure of 9.56%). It’s a pretty well known trick here that you can account for tax just by decimal shifting and adding (ex: 5.29$ without would be 5.29$ + 0.529$ ~= 5.81$ with tax). Is that 9.56% an “in practice” figure that accounts for rounding down? I’m curious where you read it.