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

No...not in relation to components. There are some who would say if you spent say 20K on components, you should get at least $5000 worth of speakers. As if to say no speaker is worthy under that. Nothing could be further from actual as opposed to perceived truth.
The only formula (and there is no magic about it) is your satisfaction.
The trouble with all this audiophile stuff... people seem to be afraid to be satisfied thinking there is always something better.

Now that can be thought of as "magic" if one believes that beneath your component table lies the sirens of dastardly temptation beckoning and beckoning with the song in the winds of "you need to spend more money"

I have the magic formula will list ASAP!!

the answer has to do with your total budget.

if your budget is under $20k, then spend 60-70% on speakers, spend 20% on the best amp you can buy, and 10% on a good bang for the buck CD player (or dac with a volume control if you already stream or own files) with a volume control. cheap digital is remarkably good. relative to how bad cheap speakers are.

speaker performance will be the dominant attribute or determent to any budget system. investing in sources is a waste if the speaker is marginal. a great speaker will make everything else much better.

if your budget is over $20k then it’s a total free-for-all on how you do it......assuming you do the speaker part right. you cannot overcome compromises in a speaker.

get the speaker choice right and all the rest is trivial.

in my mega buck system, you could plug my phone into my amps and it would still sound awesome. but throw a cheap/average speaker into my system and it would sound....cheap/average.

Here is how i break it down, 1st top priority is locatinga  speaker that you know you will love. = does not mean spending a  % of entire budget,,,
I don't believe spending over $3K ona  speaker system, as my thors cost $1600 17 yrs ago and now a  new upgraded xover will run ,,ohhh about $1K a  pair = $3K. 
a good integrated amp used,,ohh about $2K, a  excellent ohh about $5K. Tube that is. 
Source, my cayin Cd17 cost new 17 yrs ago, $1300. so you see it has nothing to do with percentages,, Find your speakers, then your amp, them source.  

Hey millercarbon,

You always seem to be a huge proponent of power cables and speaker wires having a huge impact on the sound quality. I'm not saying you're wrong, but James Randi has offered a prize of $1 million to anyone who can demonstrate that $7,250 audio cables "are any better than ordinary audio cables". This was in 2007, so I'm not sure if the offer is still active or not. If I were you, I'd try to claim that prize if I could. Not only would it net you a substantial sum, you'd also be able to crow about being right this entire time. Good luck.