I wouldn't put a powder down. I like the idea of while sheets of paper in random spots- but once you plant a few times with your seeder - you get a good idea of how much to put down. I have probably the worst - cheapest spreader out there:
you pull a trigger to open the seed hole more or less - I go by feel - and I've done it a lot. I tend to overseed when I use equipment (like a seeder with no instructions) - because I have no "feel" for what I'm putting down.
If you truly overseed you can mechanically thin the plot by rolling a cultipacker, or I would use disks run almost straight. Better to be to thin - than too thick with brassica.