Those websites are only somewhat helpful-- they usually say something like "after the last frost," and then give a month-long range for when that last frost is likely to be. Or they say "3 weeks before the last frost," which is impossible to figure if the last frost could come any time within a certain month.
So if you need to know whether to plant in February vs. May, it is helpful. If you already know to do it in February and just need to narrow down which week, it is much less useful.
(Of course a big part of the problem is that the last frost really does vary a lot from year to year, and there is no reliable way to predict this year's last frost until it is almost here.)
Have you grown them before? If you expect to do it every year, it can help to keep notes of when you start them, when you plant them out, whether they are the right size, and whether they actually survive vs. die (whether the plant-out date was right for that year.)