Real boiler pressure and its impact on t.e. and operations

Hopefully I can explain my question in a clear enough manner someone can answer it.

Unless you want to operate a locomotive, let's say a K-36 for this example, at it's maximum boiler pressure all the time (which would probably mean the safety valve would be popping on a far too regular basis), what is a typical "real" operating boiler pressure and what impact does it have on tractive effort?

For example, I would imagine working up the 4% grade to Cumbress would reduce boiler pressure a good bit unless the fireman really works hard and the injectors don't cause any problems.

What would a 10 lb. drop in boiler pressure do to a K-36's tractive effort?

What would a 20 lb. drop in boiler pressure do to a K-36's tractive effort?

What would a realistic average boiler pressure be on the run from Chama to Cumbress? Were allowances made for this in determining the tonnage for a run up the hill?