diameter of served 3/8" wire

[benz]

I'm in the middle of a similar mental wrestling match but which involves leathering instead of serving. Here's my seat-of-the-pants method: multiply the diameter of the serving twine by 2. (example, 1/16" marline X2 = 1/8"), Multiply the thickness of the parcelling material by 2, or by four if you plan to overlap it as you wind--( say, 1/32 X 4= 1/8). Now add those two eigths (of the marline and the parcelling) together to get a 1/4" more than the 3/8" of the wire. So, 1/4 + 3/8 equals 7/8". Now find out whether a 7/8 thimble will fit into your turnbuckles and Presto! you're either out sailing next month or back to the drawing board.
Hope this helps.
Ben