One plant count after emergence may not be enough to tell you about seed survival and whether the seeding rate and seeding tool did the job it was supposed to do. Seedling diseases, flea beetles, frost and other factors can influence seed survival, and unless scouting and counts are done repeatedly through the first few weeks after seeding, these influences may be missed and the seeding tool or other seedbed conditions may be unfairly blamed. Please read the updated article.
Plants counts starting around a week after seeding and going again two or three times until three weeks after seeding would be ideal.
As a guide line, count:
1) The week after seeding
2) 3-5 days after you are convinced 80+% have emerged
3) 3-5 days after that
4) After a major weather event
5) At the 4-5 leaf stage when the above mentioned factors shouldn’t reduce the plant population any further.