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.