The stock Mustang GT fuel pressure should be 40 PSI (manifold referenced). The fuel pressure should not vary by more than a couple of PSI from idle to WOT.
Note if measured at the fuel rails, this is NOT manifold referenced. At idle, the pressure will be less by the intake vacuum.
This is a technical forum with a lot of very knowledgeable ppl monitoring. Instead of asking narrowly focused questions why not take the time to explain to everyone what the problem is and what you are trying to do?
For the record, no one knows what your injector max duty cycle is because it depends upon the tune and the other mods done to YOUR car.
IMO the easiest way to measure fuel pressure is to use an ODB2 scanner and ask the PCM what the fuel pressure is. The PCM will report the manifold referenced pressure which is what really matters anyway.
The yellow top injectors are #19's. The red/pink top injectors are #21's. If your tune is for #19's, then the AF will be rich all of the time. However, the PCM will adjust for the difference by subtracting fuel from the long term fuel trim. Again this can be seen by using an ODB2 scanner. Look for negative values.