Just use calculator or a spreadsheet
Your MH hash rate / Total MH hash rate * 30 (block per hour)  * 24 (hours per day) * 577 (coins per block)

For example if you are doing 50 MH and current "global net hash" is 57.49 GH (or 57,490 MH)
then 50 / 57,490 * 30 * 24 *577 = 361.315 coins per day

361.315 coins per day * 0.00004188 (mintpal HVC to BTC exchange rate) = 0.0151318726735084 BTC before 15% commission

0.0151318726735084 * .85 (100-15% commission) = 0.0113489045051313 BTC

0.0113489045051313 BTC * $509.0 (btc to $) = 5.776 per day (btc rate to $ from http://preev.com)

There you have it.

Carlo

PS super easy to input this way into a spread sheet.

PPS take note the 577 coins per block will drop to 548 shortly.

To really get an idea how much you make or loose per day you should have a watts up watt meter and calculate your energy cost per day at the wall.

Obviously minus energy costs from profit above to determine profit/loss margin and when to move on to different coin or not mine at all until diff drops and becomes profitable again.