How do you determine how much to spend on speakers

Provide yourself with an upgrade path. Looking at todays offerings I find Sonus Faber Electa Amators w/ stands for $2200 obo. And A Proton tuner for $60. There are numerous possibilities for a serious integrated around $1200. Get a decent CD or DVD for under $500 which can serve as a transport when and if you upgrade. Buy interconnects from Wicked or somebody for $35 to $50 bucks a pair and spend a couple of hundred on speaker cables. That's somewhere between $4K and $4.5K depending on the deals you can cut.
Now when you are ready you can add a DAC or a subwoofer and
use your integrated as an amp or preamp while making upgrades to either one. This way you'll be able to thoroughly establish yourself before the disease completely immobilizes any lingering capacity for rational thinking or judicious investment. Good luck and welcome to the ward.

I use the SpeakerSpender Dart Board to determine how much to spend on speakers. So far it has never been wrong.

There is no formula, period. Figure out what works for you. It might be a brand new $200,000 horn speaker system with $1500 used integrated. On the other hand, you might find unbelievable results with $500 worth of used Spicas with a new Ongaku. Many world class musicians find their needs met by very modest gear and might not even know what brand of speakers they have.
Making pronouncements about what fraction of your available funds must be applied to the speaker purchase, reveals a naivete you should prefer to conceal.

I think it is incredible that a thread based upon such a foolish premise has survived this long. Such reflects badly on us as a group.