As most of you probably do, I sometimes turn to spreadsheets or a calculator for such calculations. I keep coming back to an old standby, though, that often makes my life easier and gets results faster: the free J.
Just this week, I had some numbers to explore, and I started in a spreadsheet. After a few iterations, I was beginning to get lost, even with comments on cells to help me recall my assumptions. One of the challenges was the inability to see the assumptions, calculations (equations), and results, all at the same time.
So I switched to J. I opened a script window and started typing. Assumptions went into comments. Calculations went into simple equations. Constants went into even simpler equations. Every so often, I'd save and run the script to see the output. When I didn't get what I expected, I'd enter the name of a variable in the execution window and use the intermediate result to help understand what was happening.
Within a relatively short time, I had the result I needed, and I also had a file that documented my assumptions and process.