Add up your turnover (or hold) for each race.
Then profit divided by hold x %.

If I worked mine out by the method of how much I risk then every year I'd be on infite %, which of course is incorrect.

If you're making 2.46% after calculating this way start jumping up and down and cheering.

eg
your lays

30/12
30/8
30/6
30/6
30/3
Hold =35, say 30/3 wins, profit = 2 = 5.71%