I asked around also.
It sounds like people use rings ranging from 8 to 10 mm. depending on the muff size of the bloodline they are keeping.
For comparisons purposes:
10 mm is commonly used for West of Englands, Saxson monks and Moorheads which are the thicker side of the muffed birds.
Oriental frills use 9 mm but they are a smaller breed then Takla.
8 mm. goes for the Short face Berliners and the Kazans.
Takla also ranges in size a bit based on the bloodline.
I would assume Ankara Sivas type birds would be OK with 8 mm while Urfa or Mardin at 9 or 10 mm.