Oh I got this... Let me go dig in my Muscle Mustangs and Fast Fords ...
Retyping this from an article John Hunkins wrote:
Calculating fuel demand, injector size and fuel pressure for any engine is a fairly straightforward affair that we've covered on several occasions, but we'll review it once again here. First, estimate the total engine fuel demand by multiplying the estimated horsepower by .55. This will give you the fuel requirement in pounds per hour for an engine with a brake-specific fuel consumption of .55. Next, divide this total fuel demand by the number of injectors (eight, in our case), and tack on an extra 15 percent for safety.
The number you have just arrived at is the target injector size. If the number is in between two existing injector sizes (and it likely will be), you'll want to go to the smaller size and raise the fuel pressure to make the difference. You don't want to go up to the next size and cut fuel pressure, because this hurts fuel atomization and combustion efficiency.
Hope that helps.
The article was actually taken from his Under Pressure Column, which he mainly talks about Supercharging... But this part should be the same for every car I believe. I hope.