Ok. Here is a summary of what I learned from using that tool.
1. Contrary to my previous belief, fort's don't really matter that much in large land battles.
Consider this battle:
What changes if we change the fort level from 1 to 5, but remove the single tank in army A?
Virtually nothing. The single tank is about as relevant to the defence of army A than a lvl 5 fort would be.
Armies with tanks and inf shielding can assault forts no problem. They will tear down the fort in the first few turns of the battle, and still have enough attacking power because only irrelevant inf died. The variance doesn't matter here because of the law of large numbers. It's probably more relevant in smaller battles.
Having more heavy units present is much more important than forts in large land battles.
In my recent game, a huge enemy stack of 200 inf, 3 heavy tanks, decided not to attack my lvl 5 fort with 35 inf, 2 ac, 1 art in it. They would have obliterated me if the attacked. They instead sent bombers. Not a good idea.
(note that they would have won if they had sent in more fighters).
OK, I get it, they didn't know what size of an army I had there. But they were acting under the assumption that they would surprise attack me and force me to divert my main army from elsewhere. Given that situation, they should have attacked with the land force anyways.
2) Naval infantry makes a huge difference.
Consider this battle:
Army A clearly outguns army B but is obliterated. Look at that loss ratio in terms of materials and cash!
What happens if we change the number of transport ships in army A?
Turns out they win the battle if they have just a few dozen more transport ships. They win with 35 instead of 15 inf, and the loss ratio in terms of material and cash quickly turns if you add just a few more.
So this is what I meant when I say that I can make much better tactical decisions with that calculator.