I had a small layout with a 0.5" elevation. What is the size of your layout? Others more well-versed in math will remind you about the rise/run issues to consider. I would avoid having anything higher than 2" incline on a layout, but that's b/c I don't want a roller coaster or elevation change issues.
I haven't done much with grades, but I would think that much of the appearance of the grade could have to do with the way you do the surrounding scenery. Are you going to be a lone wolf operator or have operating sessions or open houses. If you are a lone wolf, only you can decide if it looks right to you or not. If others will see it frequently, then a little more planning is needed. Hopefully someone has done a similar project and can give you better advice.
Good luck,
Richard
I am planning a new layout. And I am wondering what your opinions are on grades from a visual only stand point. My trains will be short, so I am not worried about trains stalling, but want to avoid looking toy like with non prototypical steep grades. But from a planning stand point, steeper the grade less room I need to get to the second level. My HO scale layout will wind its way up a mountain side (Canadian Rocky mountain scenery) in a kind of visual helix with lots of tunnels, snow sheds and bridges.