TT,
The NMRA standards are greater than what you are allowing but I haven't checked them recently.
When I designed my curves, I eased the outer track out an extra 3/8" so I have 4 3/8" centers on curves. I did this on all curves less than or equal to 72"R. On wider curves, 78"R and 82"R, I did not enlarge the center-to-center distance. I call the extra 3/8" "peace of mind".
Ed