Dave,
In spite of what you may hear, the steepness of a grade on a model layout is whatever you"re comfortable with.
If you want the rivet counter's view,,,,,. The Uintah ran its 2-6-6-2Ts on a 7.5% grade( that's 7'-6" rise, every 100'), and when the engine headed donwgrade, the waterglass would show empty, indicating that the crownsheet was dry!!!! They added a second steam dome, and lowered the crown sheet to prevent a disaster.
Oh, by the way, there was an 80 degree ( about a 11"-12" radius, in HO) curve in that 7.5%grade which was later reduced to 66 degrees (about 15"). This was 3" gauge railroading, and extreme at that.
The normal accepted grade is about 2%, but again, it's what you are comfortable with. You can run helpers, or double the hill( this is where half the train is brought up, and the loco goes back down and brings the other half up).
Pete