Mine is $300 and I also made it up. I basically did the same thing you did, set a floor and sweep anything above it into savings, and the floor thing works because it's a number you can see without doing math. The trick for me was picking a number low enough that a slow week doesn't blow through it, because once I had to dip into the floor a couple times in a row I quit trusting it. Sounds like $500 is working for you, so I'd just leave it and not overthink why you picked it.

Ah yes, the ancient art of taping foil to a console, truly a big brain fix from the Rochester game shop oracle. Next you'll tell me a rubber band and some hope will fix my joycon drift. But honestly if the numbers are real then who cares what it looks like, 80 to 71 is no joke. Just don't come back here in a month posting about a melted strip of foil fused to your motherboard.