Correct me if I'm wrong but if you break down the Heinz bet and work out the bonus for each of the individual bets it works out. You only get 20% bonus on one of the individual bets, the accumulator. The Heinz bet includes 15 doubles, 20 trebles, 15 fourfolds, 6 fivefolds and an accumulator. That's how I've been doing my bonus calculations.

If you did a 6 fold bet without enumerations then you would get the 20% on the entire winnings.