The recommendation for your area is to use premerg when the forsythia's begin to bloom. That's the bush with the yellow flowers in early spring. Beyond that, dandelion is very easy to control with spot spraying. The nice yellow flower on the dandelion even tells you where to spray!

To prevent dandelion seeds from germinating I'd apply it now, That's for seed in the soil. Dandelions grow as soon as the soil warms; just a little. I assume when the plants grow the seeds will germinate.

As far as I know the only reason to worry about when to apply is if you planted grass seed and need that to germinate before applying the pre-emergent. And that rule is when you need to stop crab grass from germinating. Going from memory the rule is to get the pre-emergent down before the 2nd or 3rd week of April. And that's only for crab grass. If you don't have a crab grass problem then there's no reason to worry. But there are other grasses which weed killers won't kill. I don't remember if the April rule is a local custom but I'm in zone 6a and as I stated above I'm not sure of the exact week..

Forsythia's fine, and jumping the gun a little is fine. Don't jump it too much as most pre-emergents have a yearly limit and shouldn't be repeatedly re-applied too often (they'll cause some negative side-effects and can be environmentally mildly problematic in larger amounts).

The most scientific method is to apply as soil temperatures begin to rise past fifty in your area: https://www.greencastonline.com/tools/soil-temperature

Very little sprouts before that. Dandelion certainly won't. If P. annua (invasive grass) is a concern of yours, it was actually a fall sprouter and the shield needed to be up in September.

What do I do? Something completely different. :-) I apply a long-lasting shield in late summer that lasts through the winter, spring, and into summer, assuring that I get coverage for P. annua and P. trivialis. Other than that, I don't really care as no other weeds are a significant issue in a lawn like mine.