The end result is you want the front tire to be straight up and down through the corner to provide the best turning traction.  Parts bend and tires flex under load; there is play in rod ends, ball joints, etc.  All this causes the tire to roll up on its side under cornering which decreases the foot print and kills steering traction.  Check out an Indy car, Formula one or NASCAR.  At times, they are running as much as 10°.
1.    However, make sure the air pressure is correct before leaning the tire in further. 
2.    Always set camber before setting toe and obviously after changing camber, toe must be reset.
3.    A good starting point is ½” from the bottom of the tire to the top of the tire.  This is easiest measured with an L square set up against the outside of the tire.  AFTERMARKET a-arms, we will typically set at 3/8 ~ 1/2" and stock 1/2 ~ 5/8".  Aftemarket a-arms, like ARS FX, Tex Tech, Leagers etc.. are larger tube and an over all stouter a-arm and have less deflection - however, most use the same upper ball joint. This is why we start with slightly less on aftermarket stronger arms versus stock.

4.    The more camber the more difficult it is to steer.
5.    Too much camber will cause a "darty" feel in the bumps.
6.    A great way to check is to have someone take a picture in the center of the corner at speed and look at the outside tire.
7.    Oval track racers will run negative camber on the outside tire and positive camber on the inside tire.  In this instance both tires should be straight up in the center of the corner as listed above, providing maximum traction. 

With oval track racers, correct camber can be fine-tuned by checking tire temperature across the tread.  TT Racers will be determined by the turns in the track.  Since checking tire pressure on ATV's is usless for all but TT, a picture says a thousand words...

Negative Camber