Look at it as you would an investment. Take your current balance, subtract your 'investment' (aka your initial deposit), and that is your profit. your rate of return would be your profit divided by your initial deposit. this would get you your 'overall return,' as opposed to a 'betting return' which could be calculated as simply your profit divided by the sum of your initial deposit and any bonus received.

for example, if i deposit $500, get a 20% initial sign up bonus ($100, therefore), and my current account balance is $750, i have an overall return of 50% ($750 balance less $500 deposit = $250 profit, divided by $500 deposit = 50% rate of return.) and i would have a rate of return from betting alone of 25% ($750 less $600 = $150, divided by
$600 = 25%.)

if you have several deposits, simply sum them and use that as your investment basis. likewise, if you've taken cash out of your account, you'll also have to adjust your basis and profits in your calculation. this is a little more simplified b/c it doesn't take into account annualized performance returns, compounds, yada, but it gets you a reasonably accurate performance return by which to measure your money management.