If you have Excel, try:

1. BTC Per Day = (B3*POWER(10,9)/B1)*(B4*B6)/B5
2. B1 = 907549524 (Difficulty Factor - obtained from http://bitcoindifficulty.com/ or http://bitcoinwatch.com/)
3. B3 = 70 (Hashing Rate in GH/s)
4. B4 = 25 (Block Award in BTC)
5. B5 = POWER(2,32) (Is a constant, is the number of integer nonce values that can be hashed with a 32 bit integer register)
6. B6 = 86400 (seconds per day)

Answer for #1 is 0.038790162 when the data from B3, B4, B5 and B6.

I add two other fields in my spreadsheet: 1) Difficulty Factor Compound Rate in % per 2048 Blocks (somewhere around 15% to 20%), 2) the value in BTC denominated in USD. The remainder of the spreadsheet I use determines risks associated with late ASIC hardware deliveries in terms of BTC or USD opportunity losses. Absolutely, don't use any of the online mining calculators to make big Mining H/W acquisition decisions... You must take into account the non-linear difficulty factor compounding rate to make reasonable computations of what your profits are likely to be. Otherwise, your estimated profits are likely to be losses...