Below is a link to a battle calculator that I made in Excel and saved to my Google Drive space. You will need to go to "File > Download As > Microsoft Excel" in order to save your own copy and edit the values. The only cells you need to fill in are the ones highlighted in light orange, the rest are formulas that update based on the numbers you plug in.

The spreadsheet can be found here:
https://docs.google.com/...V5UUllQ09rTi0xME9aaUFLdnc

A few notes on how to use it:
1) Convert Government, Readiness, Def. Bonus, and Weapons Tech percents into a decimal numbers when entering into the excel.
2) If he has alliance expenses, make sure you spy alliances and get their information too, or send double his defensive strength.
3) The middle results row "P.S. Pwr." stands for Planned Strike Power, and should mostly be what you use to land grab.
4) To error on the safe side, in case my calculations were not 100% or to account for an extra luck bonus (ontop of the 110% sent), I added another multiplyer called "Chance" to the calculation which can be changed in the bottom left green box. I have mine set to 110% multiplier, which means if the rest of my calculations were correct without the Chance multiplyer, then I would be sending an extra 10% (or 120% total of their defense).

At some point I was going to test its accuracy in the express server, but I just haven't gotten the time recently. But so far it has worked 100% for me, the only attacks I failed were my own fault for forgetting to put something into the calculator.