I dont like to see the injectors duration exceed 80 where possible unless for very short periods of time. Injector stutter and coil overheating can wreck an engine when pushed hard with high revs, but it has to be said modern injectors are very resiliant, but my professional opinion stands that if you want more power, buy a bigger injector rather than risk pushing your old ones harder. Once the customer understands that that is my view, i will of course do other than teh above if asked.

To calculate injector size required is quite simple really:

First:
Multiply your required Engine Horsepower X your BSFC = (answer 1)

Next
Multiply the number of injectors used x 0.8 = (answer 2)

Now divide answer 1 by answer 2.

Example:
500bhp x 0.5 = 250 (answer 1)
4 injectors x 0.8 =3.2 (answer 2)

250 divided by 3.2 = 78.12lb per hour

So you need to find an injector that will flow 78.12lb per hour at your given fuel pressure and your job is a complete.

That is pretty much it.
For a high comp race engine, figure on around 0.42 BSFC and a low comp turbo lump around 0.55 - 0.65, and you wont go too far wrong.

A good cossie lump can have a higher than expected BSFC and as a result can make more horsepower per Lb than some others, so bear that in mind.

Hope this helps.