First convert the price of every horse in the race into a percentage; do this by adding one to the price then divide 100 by that figure. For example, 2/1 = 100/(2 + 1) = 33.3 [recurring] %.
Then add all those percentages up to get the overround.
So in a hypothetical race where you have 6 runners all priced at 2/1, the over-round is 199.9 [recurring] %
You know where to find me if that’s not clear enough!