Remove seed tube, sit on drill (be careful if Plowboy happens to be driving ), have operator start planting at normal planting speed before you get to the first flag and continue to drive past the second flag, collect seed with cup in 100 feet.
Weigh seed in grams. (Let's say you got 42 grams in 100 ft) 42 grams/ 100 feet = 0.42 grams per foot. 0.42 grams X 69,696 feet (distance one 7.5 inch unit travels to equal one acre) = 29,272 grams per acre. 29,272 grams/ 454 (454 grams = 1 lb) = 64.5 lbs per acre.
64.5 lbs per acre X 3100 seed count = 199,950 seeds per acre.
Adjust for germination, check depth.
Note: I have also found that if the desired rate is incorrect, a quick math calculation will get the error corrected fairly accurately.
For example: I want 180,000 and I just dropped 200,000 on Notch 21. 180/200 = .9. Take Notch 21 X .9 = 18.9. Set drill to 19 and recheck.