Injector size for your horsepower on gasoline or E85, naturally aspirated or boosted. It also corrects for fuel pressure and checks duty cycle from a datalog.
BSFC (brake-specific fuel consumption) is how many pounds of fuel the engine burns per horsepower per hour. The defaults are common rules of thumb: about 0.50 for naturally aspirated gasoline, 0.60 boosted, 0.70 on E85 and 0.80 on boosted E85. A tuner with real data from your combination will have a better figure.
Injectors are rated at a specific pressure, often 43.5 psi (3 bar). Running more pressure raises flow, but only with the square root, so doubling pressure gives about 41% more flow, not twice as much.
On a four-stroke engine each injector gets 720° of crank rotation to fire, which takes 120,000 ÷ rpm milliseconds. Pulse width divided by that time is the duty cycle.