Timing Crabgrass Control After Spring Seeding

Tilled and reseeded after crabgrass takeover; grass up but not mowed. When to apply weed/crabgrass killer? The expert says seeding should have been done last fall. Expect crabgrass this season. Wait until grass mowed a few times, then apply. If crabgrass returns, kill in September, reseed, then use preventer next year.