For a SWFA 6X on a Ruger American .223? I am thinking medium.

That is what I used on my 6x/RA combo and it works great.

http://www.brownells.com/GunTech/Ring-Height-Chart/detail.htm?lid=11213
I've never used this but it could be a guide line

Thanks

Low 30mm seem to work everywhere I've tried.