Most people seem to recommend keeping grades at 2% or less. The track planning software I use turns the percentage from green to yellow at like 2.5% and red at 4%. To stay at 2% for a 6" clearance you'd need to run 300 inches or ~25 feet. With 12' you'd be looking at a 4% grade to rise 6 inches.
FWIW, a lot of people here have recommended about 7" between tracks, but I think this is to leave room for the thickness of the benchwork supporting the track and/or a bridge. If you are crossing over with just a bridge, don't run double stacks or high cubes, and your roadbed isn't that thick, you might get away with less.
Edit: I would add that I've seen people get away with up to 6% grades if they have traction tires and keep the trains short. The other thing to look at is how much of the train will be on the grade at anyone time. There are trade-offs to be made there between grade length and slope I'm sure.
