I found a formula that will do hydrometer corrections no matter what your hydrometer is calibrated for. Mine, which is about 20 year old, is pegged to 70 degrees instead of the more standard 59 degrees.
This is the formula for Microsoft Excel:
Easily, the longest formula I've EVER typed into a spreadsheet. At least it works. I've tested the results against hydrometer correction charts in "How to Brew", "Brewing Quality Beers", and "Brewing Lager Beer".
There are some assumptions you will need to know for that formula to make any sense. Do this:
1) Create a new spreadsheet.
2) Cell A1 will hold your measured Specific Gravity reading. Type in 1.050 for testing purposes.
3) Cell A2 will hold your measured temperature. Type in 120 (I'm assuming Fahrenheit) for testing purposes.
4) Cell A3 will hold your hydrometer's calibration temperature. Mine is 70, but you can type in 59 since that's probably what yours is.
5) Copy and paste the above formula into cell A4.
If you used the values, 1.050 (measured s.g.), 120 (measured temp), and 59 (calibration), then you should see 1.0610 for your calibrated specific gravity reading.
I'm building a spreadsheet to help me plan my beers and I'm willing to give it away for free if anyone can tell me how to post it to this Forum.