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

I agree with those above who stated there are too many variables. 
The greatest ones are your total budget and the efficiency of your speakers.  I have two sets of Magnepan and ended up spending a lot more on power amps than I would have spent on efficient speakers.  
However,  for the past 25 years, I appear to be addicted to Maggies.  

Try this to assess the sound in lieu of measurement gear:

https://ohmspeaker.com/news/thinking-outside-the-box-listening-outside-the-room-or-the-bouncing-sound/

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).

The speakers and room provide the vast majority of your impression of your system assuming competent amplification and source.

This is right on and my experience...

But controls of the acoustic of a room has 2 part: a passive well known one, easy to read about ; amd an active one very important and no so easy to read about and figure out...

But all that can cost peanuts with your ears and listening experiments...I transform mine without any measure, and the result is amazing on all count at nearfield or regular listening...


But decreasing the electrical noise floor of the house is very important on all aspect of S.Q. just below the acoustic controls....

I buy low cost vintage very good elements for the system.....

The more money i invest were in my homemade controls of the mechanical, electrical, and acoustical embeddings.... This is the KEY....

When you forget upgrades and listen music like never before you know you are there.....

It reminds me of house buying; the couple has an idea how much to spend on a house, how it will impact their life, etc. By the time the realtor is done, many have overbought, wrecked their budget, and will suffer relational stress as a result. So much for the plan.  

Go ahead an make your plan. Even select the picks for the system. Then, go hear them. Watch the entire plan shift and something utterly different end up in your place. Or, worse, run your plan remotely, and end up only marginally satisfied. It happens. Generic templates offer generic success, excellence at being average.  

As usual, almarg has great advice. The one aspect impinging on his analysis is that of the dramatic variability of quality of sound among components/wires/speakers. Make one wrong move on a system and you have severely compromised it. It's the reality of the performance game. 

Far more realistic to expect a nice result initially with a plan to tune/upgrade it as time goes on.