Since most expenses are static but my income is variable, I have done the calculation for how much my expenses are per day and per week. Then I just make sure I’m making at least that much every workday / workweek. All the surplus gets split between discretionary spending, long-term plans (home improvement, family trips), and investments.

I’ve managed to make it even simpler by making the investments automatic and hands-off, so I can just figure those into the per-week number since they’re also regular.