I'll try and help Clive. Look at Betfair's Racing Form and Results page, a race from yesterday Albany Race 6. If you were Dutch backing the divs read:
3.80 4.50 6.80 7.40 9.20 21 27 30 40
If you enter these figures up to 27 in a Dutching calculator (www.oddschecker.com/betting-tools/dutching-calculator.html) for 100 staked, you will get the following profit $4.07. In other words if you dutched the first 7 horses in order of price and one of those horses won, you would get a profit of $4.07 less commission = $3.80. What you need to get is at least $10 so this bet, although in profit, is poor value.
Remember too that you haven't got time to be entering figures manually into a calculator as they change all the time so you need a bot that only fires a bet if your desired profit is met.
Interestingly, in this particular case, if you dutched the first two or three horses there was good value but that's another story.
Hope that helps, RP