The numbers displayed are the standard max attack/defense for the units on both sides of the battle. To do the maths correctly, just add any changes your upgrades/strategy incur multiplied by the number of units. e.g. if attacking with 30 tanks + general with upgrade, defense will be 8x30 + 1x30 = 240 + 30 = 270. (just add the 1x30 to the 240 already displayed)
If your strategy was also Relentless Attack an additional 60 attack would be added to find the max attack of the stack.
Don't trust that having just +1 over will be enough, as this is only the max attack, with the actual attack done being quite different.
And it won't show any defense bonuses the enemy has on the statistics either.
A battle mechanics explanation is here although it does not include the actual calculations:
http://atwar-game.com/home/faq.php?faq_id=17