Jon's advice is very good and the proper way to calibrate seeding rates. Problem is, you need to do it each time you change seed lots or varieties - time consumeing, but save's big $$$ with seed costs.

As posted above, consider the SI belt meters. They can pay for themselves in one season (if you do a few hundred acres). They are with in a few 100's not 10,000 as the JD meters can be. With your current set up, it just can not be fine tuned with the slide / meter set up, and get down to a few hundred seeds/a differences, and less money then a monitor if you need to buy one. . . .

I started with a scale on my drill - at the time, it was the best way to go and paid for itself in one season with RR beans. Now I use it primarily to keep track of seed level in the drill. The SI Belts (direct from them or Shoup) would be a better option then a scale now . . .