Acceptable grades

[faraway]

I did some more measuring and calculating. The best I can hope for is 4%. I guess I need to do some more thinking.

As discussed already is 2% the "standard" grade "standard" trains should master. 4% is not impossible. Many layouts have somewhere such a steep grade but the risk to fail is likely.
You need to do testing. Get some flex tracks on a long support, put your trains on it (light engines and poor running cars) and raise one end until the engine stalls. If you plan to have a curve on the grade put it on the test track too, it is important.