There doesn't seem to be a minimum distance I could find, but this seems really close. I think it would be okay and have no issues draining since the lowest part of the trap arm is below the tub drain line, but this just looks odd being stacked so close. I already need a riser under the tub, just trying to keep the riser as low as possible. If the math works out, I'll end up with 1" to play with so I think I can lower it that amount. Any thoughts?