"or for every 100 inches you can go up 1 inch at 1% to 4 inches at 4%."
I don't know what method you'll be using to get the elevation you're looking for but don't forget to include in the overall length the distance needed to transition from a 2 0r 3% incline back to level, 0%, incline.
I used the Woodland Scenics 3% risers to achieve the elevation I wanted to get a bridge into my small layout and by the time I got back to level I'd used another 6" or so, on each end, up section and down, which reduced the overall space available for a bridge on the layout.
If I just left off where the risers ended I had what appeared to be a "launching ramp". I'm sure with a smaller grade, say 1%, this would not be a very significant problem.
George