I use a weird way of determining a height of track. 1 train height is equivilant to the train clearance plus 10mm. This is around 8.5cm for my pruposes.
How I work it is thus: Generally the lowest point is classed as 0 train height - the datum. Each level has it's own datum. Then if the track rises 8.5cm it has reached 1 train height, so I usually mark that next to the track. If it hasn't I guess it's height, ie .4 train height. Generally I leave around 14ft per level that works roughly as 1:40 - which I consider very steep.
So grade determination is rather haphazard. Yet I have used this for the last 16 years , and I have drawn 100's of plans, and 1000's of doodles. Some plans have been built - so I know they work. One instance it didn't work - I had the track rise 2 loco heights in the space I now allow for 1. That layout gave locos too hard a time up hill - it just didn't work.
If I do calculate specific grades it will be shown in 1:40 - a ratio, not a percentage.