Usually I tell people to look for 70-90% coverage, just a little bit of brown showing through.
Sometimes I suggest when they are starting if they know how much area their machine is supposed to seed in one load, to mix everything according to what the manufacturer of their machines advises and then to measure the area they actually spray in a load. For example if someone has a 300 gallon machine that is supposed to seed 4000 sq ft per load and they measure how much area they sprayed in a load and it turns out to be 4000 sq. ft. , they are probably spraying just about perfectly. If instead they sprayed 2000 sq. ft, they are most likely applying it too heavily and if they sprayed 6000 sq. ft, they probably are not applying enough material. I would say that spraying a little less than the machine is advertised is probably ok, In other words if you actaully sprayed 3500 sq. ft. that is close enough.