Is there a general rule of thumb on how much defense your league fortress needs?

My league's fortress was recently taken down by a powerful opponent - not sure of his total attack points, but we had about 41 million def, and were holding out very well against a constant barrage of small and medium attacks, then a single player came in with a massive force and took the fortress down- 10k dragons, 63 Great Westerns, 7k necros, 12k warlocks, 45 Fallen, just to give you an idea. I'm trying to figure out from what the battle report tells us approximately how many offense points he had and how much def it would have taken to defeat him. 

The attack report is a little confusing to me- it reads def losses at -5593.5K (which I assume represents only my personal troop losses), but attacker losses at only -12.1 (that's right - no "K", even though I see he lost thousands of dragons, necros, and warlocks). Can someone help clarify?

Thanks much.