Ron and I always have gone with 16" for the ear tip to ear tip as a benchmark when trying to estimate inside spread. I'm sure there are exceptions as well but with many discussions with our taxi(Derry) over the years this is pretty much the average number of inches...IMHO.
I do 16" for younger bucks but the older bucks I use 18" but I could be wrong. Not like I have anything to compare either of those spreads lol.
I tend to shoot basket racks lol