I made this spreadsheet in Excel to calculate combat odds. I'm using it to balance out my custom units.

It might prove useful.

Note: Requires you to enable macros. I had to write a custom function to properly deal with First Strikes.

Edit: Fixed bug where odds didn't compute when Defender has more first strikes.

There is an error in your calculator.

CombatI-IV bonus for defender does not multiply whole defense.
It's additive just like other defensive bonuses (culture, city defense etc...).

Another Error:
In the Excel, B15=B6/B5*(B7*1+B9)
Acturely, it should be B15=B6/B5*(B7*(1+B9))