I ran a JD 750 with SI meters and would not run a drill without them.  As long as your seed size is within the window you will have an accurate population.  If you get bigger seed you can have problems and if you are smaller than the book suggest you can have double drops.  Within the suggested size you set the population with the sprockets and you will be at that population.  Our checks usually put us with 500 per acre.

Gunner