Although not a formula, the attached chart plots the capacity of various fuel pumps at various voltages against the fuel capability of various injectors at 2 or 3 boost levels (usually 15, 20 and 25 psi) and 100% duty cycle. Where the curves cross is the maximum flowrate at that operating pressure that a certain configuration can provide. Total gph is on the left axis versus rail psi on the bottom.
I don't know how readable this will chart come out but...
For example, the lowest capacity fuel pump curve on the chart is the 3270 (TTA/SY/TY/FBI etc.) pump at 12.5 volts. Stock injectors at 100% dc are also plotted with 15, 20 and 25 psi boost. At 25 psi boost, the curves cross at 28 gph (approx 336 hp) and 64 rail psi (39 base psi). So, if you try to run 45 psi base with stock injectors at 100% dc and 25 psi boost with this pump, a fuel pressure gauge will show that the total pressure will drop off to 39 psi.
The same stock injectors at 25 psi boost with a 340 pump at 12v are good to 37 gph at 90 psi (65 psi base). This is an extreme example but chosen to illustrate the chart.
As another extreme example, 83 pph injectors (100 % dc) at 20 psi boost with a single XP+ pump at 13.5 v are good for 70 gph total (840 hp) but only 50 rail psi (30 base psi).
This chart assumes gasoline.
No consideration has been given to airflow requirements, but one can asssume 11:1 AFR or so, this will llink fuel flow to airflow, and give some places to look at on a compressor chart.
