A very basic question, but one which is leaving me a little perplexed!

Example:
Betting Betfair SP min $30.
100 races laying 5 selections per race
Profit $100.

a) Do I say ROI is: 100 x $30 = $3,000. Percent return = 100/3000 = 3.33% (ie use the number of races)

or

b) Do i say POT is: 500 x $30 = $15,000. Percent return = 100/15000 = 0.67% (ie number of bets).

I have started laying Oz races for August and I am showing 2.46% return using calculation method (a) above and I am trying to find a measure to see if this is satisfactory. If I should be using calculation method (b) above then what I am doing is hopeless!

Many thanks ... Fred