How wide should the black wheel centres be?

[Invicta]

I have just received my new rims for my 1949 model 18 and want to get the rim centres powder coated black, and then pin striped in gold. Can anyone tell me how wide this black strip in the centre of the rim should be? My 1950 model 18 still has its original rims but there is not enough of the black paint left on the rims to measure accurately. Thank you.