Divide it by two and add two. If you end up with an odd number, round it up. For example, my WB had a 54" girth and a 30" dressage girth. 54/2 = 27 +2 = 29, so a 30" girth.
Not fool proof, but normally about right. You need to make sure the girth is long enough that the buckles are well clear of elbows