Do you have any advice or experience? I’m going to try tripdogs method by hand if I can’t figure something a little faster out.
Too bad I never took any pics while doing that (I need both hands to work) but here's how I proceed :
01 - remove all the indexes from the dial as they could be satelized while the dial will be in rotation
02 - sand a bamboo skewer to almost the diameter of the dial's center hole ... try to keep it just a little larger than the hole ... and cut it so that it exceeds about 2 or 3 cm each side of the dial
03 - push the bamboo stick "in force" inside the dial's center hole ... if "in force" enough the dial will stay perpendicular to the axis of the skewer
04 - get 2 little blocks of teflon/metal/etc ... any material on which the bamboo skeer will rotate without too much friction
05 - put a thin aluminium oxyde grinding stone on your dremel (see pic below) ... use a flexible shaft if you have one too make control easier
06 - put cotton glove fingers (cut from a glove) both on your index and ring fingers ... you'll use those 2 fingers to hold the skeewer on the blocks ... cotton cause you want the skewer to rotate easily
07 - put a leather glove finger (still cut from a glove) on your major finger ... you'll use that finger to slow down the dial if needed
08 - start the dremel ... low speed
09 - put the bamboo skewer on your blocks, with the dial between the 2 blocks, hold the 2 sides with your index and ring fingers
10 - start grinding ... the grinding stone will both rotate the dial and grind it ... use your major finger to slow the rotation if needed
Grind slowly, without applying too much pressure with the grinding stone, and check repeatedly how much material has been removed.
Proceeding that way you should get a really good result (almost perfect circle) and much faster than sanding by hand only.
When you're about too get the perfect size, just finish by hand with fine sand paper.
Hope this helps