Re: Ludwig ATLAS Mounts
8-14 would be fine for sure. I use them on a 10 & 12. LOVE LOVE LOVE em. No RIMS wiggle. Sturdy hold(finally no sagging on the L arm). Great tone. Such a huge win for my kit (mine was a first year Keystone with the bands).
Get them and don't look back. Pretty soon you'll have them all over the kit!! Just verify your hole spacing first.
As for positioning on the L arm, generally the higher you go, the better resonance you will have. Low on the arm will choke a bit more. I have mine fairly high and they sing.