How do I budget to hit a specific savings rate?

Work backward from the rate, not forward from leftovers. Decide your target – say 20% of gross income – calculate the dollar amount, and treat it as a fixed bill paid first, automatically, on payday. If 20% of a $6,000 monthly gross is $1,200, that $1,200 leaves for retirement and savings before you budget anything else, and you build the rest of your life around what remains. This 'pay yourself first' approach guarantees the rate instead of hoping for it. If the remaining money doesn't cover your needs, the savings target was too high or your fixed costs are too high – adjust one of those, not the savings. Count 401(k) contributions and any employer match toward the rate. Automate the transfers so the target is met whether or not you think about it.