Conga Head Size

[Snake]

I want to use the Evans Tri Center's on my conga drums but I'm not sure if I need the 12.5" ones or the 11.75" ones.

My drums are 12.5 inches wide on the outside and 11.75 inches wide on the inside with a 3/8" wall thickness. The top of the drums are tapered from the outside - in.

Thanks to anyone who knows which one I need.

Snake