Late summer is after the heat breaks for YOU in your area. If you live on Hudson's Bay, you could probably do it now. If you live south of Detroit, you should wait until the end of August. Usually the reason for seeding in late summer is that there is ample time before the first frost causes dormancy. It also gives you time to evaluate the coverage and add more seed in a few weeks if necessary. Clover should not need to be reseeded, because it reseeds prolifically.

As for covering with 1/4-inch of topsoil, instead use 1/4-inch of compost. Adding topsoil when you have good drainage already is going to mess up your drainage possibly resulting in water in the basement. Note that as clover reseeds itself (prolifically) it does not cover itself with 1/4-inch of anything, and it grows just fine.