Hello, did you know you can change the number of the dark lines shown for the 3d grid (Expand the numbers shown), meaning increase it to 30 or 50 dark lines (Feet), or whatever if you wish, with inches still used (As I instructed) for the light lines. This is all done in the File>Settings.
Which should work far better for you than attempting to use the lighter lines for feet. Or learn to work in metrics, using meters for the dark lines if it is a large object.
AC3D settings are very flexable for size and view angle (3D>FOV)