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

Magic formula is simple.

Spend right up to your limit...or to all you can afford...and you'll be there.

yes, it is about 3% of your yearly gross income or $15,000 whichever is less. 

To me, speakers have the biggest impact and variation on sound.

So, about 1/2 on speakers, then work out the rest from there.

I had such a clever response nearly completed when I left the page and lost it all. Now I must to condense the wisdom. Prepare yourself.

If you had $10k to spend,
( I'd suggest seeking financing for an $10k. haha-joke).

I will preface my remarks with full disclosure that you are reading a recommendation by someone who is enjoying his 2nd double of  Jethro T. Boots Kentucky Whiskey. CVS $8.99. Really tastes about the same as the $80 Japanese whiskey I had last week.

As the acknowledged weakest link in audio is the speakers,
you start there. Spend 6-12 months investigating what you want.
Find what you like. Not what somebody else likes. $4k is the max
you spend here. Leaves $6k. I am pretty good at math.

Then you will know what type and how much power you need. 
Your two brands to choose from are: Pass Labs and Line Magnetic.
If your speakers require massive power then I will offer a 3rd choice
for amplification. Magtech by Sanders. 

You buy everything used. From the original owner. Under 1 year old.

For a source-Stream with your own computer. Hardwire ethernet to your DAC to your router. Buy a Preamp/Dac combo for $1k. $5k left.

Pay $700 for a lifetime subscription to Roon. $4,300 left.

Hire an electrician to install 2 dedicated runs to your HiFi.
The cost of this will vary. Include the ethernet line. I did 
it for $350 at my house. Likely you will spend $1k.
$3,300 left.

Cables.com.  Buy used interconnects. Buy Kimber cables. 
Budget $1k. $2,300 left.

Buy 6 each  2' x 4' absorbing panels. Cost $60 each if you buy
new or $30 each if you make them. If you have a hard surface floor
get an area rug. Deal with windows with drapes. Cost $500.
$1,800 remaining. 

If you have LPs, buy a used Rega TT and a Schiit audio Phono stage.
End of original $10k.

That's all folks!!

Yes.
Spend $nil on CD player.

Don't be a cable chump.  Don't allocate much to cables until system spend gets to $10k.  Cables can never add as much as active components at this level.  Don't buy used cables, they deteriorate over time.

Yes.  If you don't have $30k for power amplifiers, choose high-efficiency speakers.

1970s Linn formula:   Budget $n.  Spend $0.8n on Linn front end.  Leaves $0.2n for the rest of the system.