you can set the grid snap from 1/8 (default) to 1/64. You can also use direct entry. Grab an object handle & jiggle it to get the dimension fields to appear and enter any number you want, <tab> toggles dimension fields (len/width)
Let's say I'm making a 4" x 6" rectangle. It's solid but I need 4 holes in it, each 5/16" in diameter. The holes are 1" in from the ends, and 3/4" in from the other two sides, basically one hole near each corner. My current method is to make the rectangle, then make the holes. I then align one hole to one corner, adjust the grid snap to 1/8 then grab the hole and get it close to position using the arrow keys. I then have to change the grid snap to 1/32 and nudge it into place. Since I want it centered 1" and 3/4" from the edges respectively I have to divide 5/16 by 2 which ends up being 5/32 and then nudge it 5 times each way with the arrow keys.
Nudging? Make the circle a nice, even size and position it using the 1/8" snap. It should land PERFECTLY on the grid. Once you have it positioned, press ALT and resize the circle to the correct diameter. This will change the diameter but leave the center alone. Naturally you have to go down in snap as you change the size. But you can type in any X/Y size and as long as you had the ALT key down only the diameter should change.
