Is the problem seeing where you have been? If so, a foam marker helps me see in some no-till fields, and is a must for spraying. Just a cheap 12v air compressor(cut the cigarette plug and wire like any 12v), a plastic gas tank with a tire valve stuck in the bottom, and some hose run out to the edge of the drill. Put a couple oz of foam per gallon of water in the tank to about 2/3 full. Shouldn't take long to pay for itself. The air compressors dont last long(~60 hrs), but they are real cheap.

Usually the book will get you fairly close to right rate, but it just takes practice to get it right.