The best information about when to apply I can find is to "apply this as a spray just before the annual weeds start to germinate" but then about 3 days after application you need to water if it does not rain.
To spend that much money on a product to control something on that small an area would convince me to learn to live with whatever.

I have read that the forsythia is a good indicator. Apparently these chemicals disrupt the germination process, but do not kill seeds.

I have read to put it down when the crocus bloom.

I'm in zone 5 and my perennials are all starting to come up already. Not sure about annual weeds, but I think the window is fast closing on putting down pre-emergent unless you are quite a bit further north than we are. We use corn gluten meal, and while the jury is still out on its effectiveness, we already put it down a week or two ago.