Yes a dial printed would work well. The amount you turn will depend on your rail and obviously the magnification as the dof will change. Even on my electronic rail I've never calculated the increments, mainly because I've never had time to calculate it all but also, because I've done a few now I sort of know how many steps are needed for a certain distance.
I've never been able to figure out how to get the knob off in order to put a disk on (I'm open to ideas).
In the meantime, I've fixed a paperclip to the body of the rail so that it indexes against the scale on the moving rail.
It's not perfect, but it's WAAAAY more accurate than trying to eyeball either the unindexed rail or dial.
I may have to just make the disk with a split in it so that it can be twisted enough to slip over the adjustment spindle.