the 20lbs of pressure is the key bismark. when i do free hand camo, i spray the whole kit the lightest camo color and progress to the darkest. i never use over 15 psi during this process. anyway, i go ahead and get all the paint on the kit like i want it, not paying much attention to the demarcation lines. i do this with about a 60% thinner to 40% paint ratio. i use an iwata HP-CS and it tends to like this concoction. the omni will be a little different as the tip and needle is of larger size. when all the shapes are done and i'm satisfied with it, i mix an even thinner ratio of paint (70/30) and dial my psi down to around 10, sometimes even lower, and move in really close to the surface. and with each color of the camo, i begin the long and arduous task of "cleaning" up the demarcation lines. the low air pressure, thin paint, and close proximity alows you excellent control over just how tight you want the demarcation lines to be. the lower the pressure, the thinner the paint, and the closer to the subject, the tighter the camo pattern. this will ofcourse take ALOT of practice.
below is an example of my freehand camo efforts done with the above explanation. for sure not the tightest lines i've seen, but i was satisfied overall. Plasticmod can produce some of the tightest camo lines i've ever seen as far as freehand goes. he's in the members list here, you can look him up and he will gladly explain his methods. good luck.
PS if you get frustrated with the whole freehand bit, try a "soft" mask. this works really well. i used to sluff off soft masking camo patterns because i thought it did'nt look realistic. after doing it both ways, the freehand is still enjoyable to do, but it can quickly turn in to a tedious experience. one slip up and you gotta break out the other color and clean it up. with a soft mask, the process is the same, work from light to dark. i use elmer's tak rolled into a "rope" about a 1/4 inch in diameter. the larger the diameter, the "softer" the demarcation line. mask off each field and use the "rope" for the edges and spray away. this too can be a little tense, but much quicker. it looks good too. good luck. later.