As far as putting them out, you can't just go by the calendar... that's why they were giving you a range of dates, because every year is different. I start the seeds by the calendar, because in February you can't know what April will bring. But in late March and early April, I am watching the 10 day weather forecasts like a hawk. (And some times 15 day --www.accuweather.com has a 15 day forecast).
That helps me figure out to do, and when to be hardening things off and when to be bringing them back in.
By late March I have trays and trays of tomatoes and other stuff that could go in the ground whenever the ground and weather cooperate. I do a lot of bringing trays in and out in those last weeks.
I also have a pop up greenhouse, that I got super cheap at a discontinued sale at big box store. It is NOT very good, not at all air tight, but provides some protection. And I have a regular metal shelving unit that I wrap in plastic to make a mini greenhouse (with velcro down the front for access)... All of this is just to get the seedlings started being used to outside temps with some protection, in those last weeks.
(And by then the 16' under grow lights is way too full and I have to have somewhere to put things!)
(It's a kind of craziness, I know
-- it all sounds kind of bizarre to me as I write it out, oh well)