This site belongs to Merovech, formerly of Ceti; now resident of Epsilon.
The following link is to a Battle Calculator that should work. I found it on the Internet, and fixed it. I also updated the shield strengths based on the recent unit updates.
To use it, you set your technology level. You then manually input the Power, Armour and Shield (PA&S) for the opponent's unit. As I've said before, have a Scout or Corvette (“scout”) sneak to the enemy fleet and run through the attack sequence to get these AP&S values. Abort the attack and retreat the “scout.”
For added covertness, sneak the “scout” to a distant astro in the region, then move to the target astro (if possible). When you see the fleet strength, copy and paste the data into a blank document (e.g. Notepad) and retreat. When your scout is outbound, you can then fat-finger the data into the calculator.
One advantage of a Battle Calculator is that it allows you to test assumptions. For example, are Fighters more efficient than Bombers—or Frigates? (Answer, 1 FR costs 80, which is 16 FTs. In a same-tech battle, there will be 10 FTs remaining. Thus, FRs are inefficient.)