You got it Mike. I tried 1/2 ounce seed on one sqft of low-quality turf and covering it with one pound of screened topsoil. It was on October 13, 2011 and temp was 65. Rain and cooler weather expected in the next 7 days.
I calculated the seed was about 30 pounds per thousand sqft.
I calculated the soil layer at about .16 of an inch. (about 1000 pounds per thousand sqft.) If a cubic yard weighs 3000 pounds that is about a third of a yard per 1000 sqft.
I do not normally topdress--no experience here.
And Smallaxe, I did try covering seed with grass clippings, plus fert, plus raked-in. Results were good, but double fertilizer, and 4 times the usual amount of seed, and rake-in, resulted in the best and thickest grass. Photos coming soon.