coins a day = A * B * C / 4294967296 / D * 1 000 000

where:

A = 86400 ( The number of seconds in a day. This is a constant. )

B = reward in coins from the found block ( We take this value from any pool where the coin is mining, or from the first post, if the developer pointed it there  Wink )

C = your mining speed in Mh ( in the end of this formula " * 1 000 000" return our "Mh" in "h" )

D = diff network ( The difficulty of the network. It is better to take a day diff. This value can be taken from different sources. But the easiest way is again with the same pools where the coin is mining - of course it will be approximately, but this is what we need )
 

4294967296 - This is a constant - it's a  2^32=4294967296 ( You may check it with calculator     )

For example (for now):

My speed in algo "C11": 444 Mh
Reward for block:          15 coins
Diff:                             6500 ( Spread around - 3800 - 18000, but more to 6-7 k )
coins a day = 86400 * 15 * 444 / 4294967296 / 6500 * 1 000 000 =  0.13398 / 6500 * 1 000 000 = 20.6

Yesterday diff was about 4000, and I mined 33.5 coins (according formula), and 32.2 coins (according to pool). Bingo!

This very simple to use with almost all coins!