Where you start has little effect on runs except that after a couple of coats you need to remember where you have sprayed and where you have not. An old time painter told me when you get a run just chase it all the way to the bottom with the gun, LOL.  Fixing the over spray problem is fairly easy too, you can take any of several routes or combinations of them. Probably the most important  fix is doing the work correctly to minimize overspray on important easy to see parts.  I  start an the hard parts, underside, the shafts etc and finish each coat with what’s on top and easy to spray, this way you are painting over the "dust" instead of it being over the fresh painted parts. Second easy and low buck thing is equally important, ventilation. I have one box fan in my "booth" and it moves plenty of air to keep overspray from being a problem. Painting with no air movement is very difficult.  Third thing is your gun. An HVLP of quality plenty good for tractor painting was less than $20 at HF yesterday on sale. HVLP process helps a lot and will cost you net zero by the time you put on the tractor parts what would otherwise be floating around in the air.. Fourth thing is again planning, a little over spray on castings is unnoticeable to all but those who carry a magnifier but on the hood and fenders cowling etc it can look bad so you remove them and paint them separately. Come to think of it if you are not going to remove them why would you be worried about overspray to begin with? LOL Fifth thing is the paint mixture which can be fine tuning or a major problem depending on how far it is off to the hot side from ideal. When your mix is on the hot side, meaning your reducer and hardener speeds are fast for the temp you will have more problems with overspray droplets that cant "melt in" to the wet paint on the parts, when off to the extreme you have a real mess on your hands. Point being You do need to pay attention to the temps and the speed of your reducer and hardeners.