There's A, S, then SS. It's not just the IP that counts in your overall rank, there's also the number of kills and towns occupied. So you can either finish the mission fast, but not kill everything that's on the map, or finish a little slower but conquer every town en route to the goal. But when it comes to SS ranks, though, I'm pretty sure (not exactly 100% sure) that you have to finish in the alloted IP or less, AND have every town occupied/enemy eliminated.
The exact IP values are listed in
this guide, the one that selkirk posted. For example, in
Chapter 2, the very first mission must have you finishing in equal to or less than IP 474 in order to get an SS rank.
You might sweat over it the first few times, but once you get the hang of it, getting an SS becomes easy. It's only a bother at the start because of the relatively low mana supply, compounded with the need to "reinforce" your troops.