How do you figure out what's actually yours to spend as a freelancer?

I'm launching Nett on June 16. It's an iOS app that tells freelancers how much of their income they can actually spend after taxes, VAT and social contributions, across six EU countries.

Before launch I'm curious about one thing: how do you work out your "safe to spend" number right now? A spreadsheet, a fixed % set aside, a gut feeling, or you just check the bank balance and hope?

I freelance too, and for years I did the messy mental math every quarter. Building Nett was basically me automating that.

Curious what your method is.