I'm throwing this out there to get some people to give this a shot and see what it does for you. All this does is allow you to enter operating conditions (shown in blue) and have it spit out some values. Basically, the calculated Start of Injection value is based on starting at a point when there will actually be sufficient time to inject the charge before the cut off (which is 5° before the spark fires).
Being that a lot of the tuning for this is strictly "try and see", this is intended as more of a visual to make sure that you aren't starting the injection too late for the given conditions. Nothing will fix low fuel pressure, and you may run into issues by moving the injection window in a certain direction.
This one is set up for factory LT1 injectors in a C7 (just added one for LT4 as well). Short Pulse Adder is not taken into account because ultimately, it only amounts to a fraction of a degree of crank rotation, so I consider it inconsequential for the purpose of this spread sheet.