Freelancers: How do you calculate "Safe to Spend" when income fluctuates?

Freelance income is a psychological nightmare. You make $5k one month and $0 the next. Every time I want to buy something, I'm doing mental math: "Okay, rent is due, that client hasn't paid the late invoice yet, and I need to save 25% for taxes... so can I afford this $50 dinner?"

I ended up coding a simple tool that just gives me one number: what is safe to spend today.

How are you guys currently calculating your daily spending limit without going broke before the end of the month?