Firstly, there are many casual guilds running Kara - so you can just factor in 25 man raids at this point.
Secondly, a lot of casual guilds that are doing Mags/Gruuls are running 4 Tanks (not all of them Main Tanks). My guild has 2 MTs and 2 Feral Druids for instance - not unusual. So that’s 16% for our guild - just for the core 25 man team. We have more people running tanks than that as we have additional groups running Kara.
My 18% figure is just a guestimate and maybe I’m overcooking it the other way but I think your 12% is way lower than the real figure will turn out to be.