At 220lbs, a stock rear shock is ok, extending it 4"-6" is not dramatic.
But, a custom shock will obviously be better.
The fork springs will definately need upgraded at your weight.
As for handling, if the forks are perfect and the rear is not, or vice versa, then the whole ride suffers. Both must be correct.
For suspension sag, it is actually simple.
35-40mm of sag is good for the street, and 30-35mm for a track, as tracks tend to be smooth, and the lower the sag number, the firmer the ride.
That sag reccomendation will have the shock functioning as it should.
Essentially all you're doing is adjusting it to Your weight, and so that the bottom third of the travel is compressed "sagging" under the weight of the bike and rider. That way the middle third of the stroke/travel is where the suspension works. And, the upper third of travel is to be your over-rev, the extra travel needed for bumps or potholes.
Compression is the firmness the shock compresses at. Rebound is how quickly it returns to it's sagged position. And Preload adjusts heigth and a certain amount of "preoaded" tension.
Shooting for set sag numbers is only 3 seperate adjustments. Each done seperately, so you can feel what you changed.
Basically, with the bike upright(have someone hold it for the rear), you want to hold the brake and push down on the forks and rear seat as hard and fast as you can.
You want the shocks to compress and rebound back as quickly and smoothly as possible, without being too soft, or too firm.
Correct sag numbers gets you close, then it's ride, stop, turn a half click, ride, adjust, etc.
Even just wanting straight line performance, sag is important. As it keeps the tires planted for traction and handling, both of which are very important for braking as well.
I am no guru by any means, but am good at making my bikes, and others bikes handle much better.
I always get a kick out of making a few small adjustments to someone's bike, only for them to be amazed, that just a few clicks made a night and day difference.
Still don't quite understand? Don't worry about it. I assure you that the lightbulb will come on, and when it does, it's going to take your riding to another level.