Gentlemen,
The Holy Grail of Injector Voltage Offset:
http://injector-rehab.com/shop/lag.html
If you look through this you can see how different injectors act under varying voltages. You can compare the Bosch v Accel and RCs for example. With longer opening times you get less fuel for the same calc'd pulse width. That's why it's important to get this offset right since it makes tuning fuel more accurate. If you have ever felt that your car has idled roughly after a long hot run, this is one of the reasons why. Once you modify this parameter, you'll need to re-check your fuel tables by datalogging.