This is how I came up with this formula.
If you do the math, you can only get 96 turns a day. If you are successful with all 96 turns (which is unlikely) you would get a maximum of 96 Evil points a day. This is equivalent to 384 total EXP points from their Evil Points. Keep in mind, they will lose several of their units by attacking. On top of this, they are actually attacking the "Good", which is our cause, rather than camping to build their army. And thirdly, they will be getting attacked, assassinated, and what not due to them advancing in levels.
On the flip side, a camper who uses the "recruiter" properly at Level 1 will be gaining 330 units per day. This is equivalent to 330 EXP points per day without having to do anything but sit at level 1. They don't have to worry about getting attacked, assassinated, or anything else so long as they continue camping. Plus, your turns accumulate greatly while camping, so when you do decide to start attacking, you've got thousands of turns to make up for the lack of EXP points.
So this is what it boils down to.
330 EXP points per day by population, all ya gotta do is camp.
or
384 EXP points per day MAXIMUM by Evil points, and ya gotta worry about Attack, Defense, Spy Offense and Spy Defense the whole way. The 384 EXP points are only possible if you have 100% Success in ALL of you're attacks, and they would have to be attacks that give you Evil Points.