Is there a magic formula for spending on components in a system

Percentages have gone up and down I’ve the years as I’ve upgraded this piece or that. And, I use new prices to calculate tough many pieces have been purchased used. That said: 20/30/30/20 percent, for source/amplification/speakers/cables. There’s no turntable in my system—it would certainly be more for the source if I was into vinyl.  And cables are usually bought used or on sale...again, I’m comparing list prices for new equipment to come up my magic formula (hey, at least it’s easy to remember).