Its all about risk and return.

Profit is the return for risk.

If you make a back bet of $30 on a horse, the most you can lose is $30. Do that for 100 races and you have outlayed (risked) $3000. If your payout is $3100, then your $100 in front. Therefore your profit as a percentage of the amount of money you have risked is $100/$3000 x 100 = 3.33%.

Now i know this is pretty simple stuff, and I'm sure I'm not telling you anything you dont know, but working out profits on lay betting is exactly the same. Your profit calculated as a percentage of the amount you have risked.

If you lay 5 horses in the one race, each to a liability of $30, only one of them can win, so the most you are risking in any one race is $30. So if you end up $100 in front after 100 races, then the calculations are exactly the same as above - 3.33%

the only difference is with back betting, your putting your $30 on up front, whereas with lay betting, you only put your $30 up if one of your selections wins.