Why a flat percentage for tax misleads you
The most common rule for tax money is "set aside a quarter". It is easy to remember and wrong in both directions: in the good months you tie up money for nothing, in the thin ones you find out in March.
Updated
A percentage is the average of a year that has not finished
When you pick a flat percentage you are picking a whole year out of a single number, in January, when you know neither how much you will bring in nor when. If the year turns out the way you assumed, you were lucky. If it does not, the percentage does not correct itself — it sits there, reassuring, until the real figure shows up on a return.
The problem is not that the percentage is too high or too low. It is that it is a constant laid over something that is not constant, and that its error stays invisible until the day it matters.
What it should do instead
A correct reserve starts from what happened, not from what you assumed: the income recorded so far, the deductible expenses recorded so far, and this year rules applied over them. Add an invoice and the reserve moves. Delete one and it moves back.
The practical difference is that the answer carries a date: "this much, on what we know tonight". A flat percentage carries no date, which is exactly why it feels safer than it is.
A reserve is not savings
Tax money is not yours and it is not a goal. It is an obligation that has not fallen due yet, and if it sits in the same balance as everything else, the balance lies — it shows a sum you might rely on when part of it is already promised.
That is why "what is left" deserves to be a different number from "what is in the account". The first is a decision; the second is a statement.
What stays with your accountant
An estimate is not a return. Whether an expense is deductible, how a kind of income is classified, what to do with a case that resembles nothing else — those are professional judgements, and an app that pretended to make them for you would do damage exactly where it is most expensive.
What a tool can do is get you to that conversation with the figures gathered and the question already framed, instead of a pile of statements and a hunch.