I guess the accuracy of such solutions vary by jurisdiction; I just received my tax return for 2025 in Norway.

The sum owed I had calculated at the end of 2025 was less than 2% off from the sum our IRS equivalent came up with.

Their sum was the most favorable to me, though - they had adjusted a deduction I qualified for last year which I had missed.

This level of accuracy is down to our IRS knowing just about all there is to know about our income, assets, debts &c of course - oh, and on there being fewer loopholes in our tax code...