Almanac uses the 30% chance date. Long time ago I saw a chart or calculator that listed multiple dates and gave a percentage for each. I think early April was 50%.
Of course it all depends on hardiness. I can plant peas any time now. Just started lettuce and leafy brassica a few days ago and they'll be in the shed for a month or so.
Noticed something I had read a long time ago but forgotten. Heat mats are only good for a certain rise in temperature above ambient. Since I'm controlling them with a PC connected controller, I can keep a log of temps. On our recent nights in the low 20s, the tray soil temp got down to the mid 40s so they're good for about 25 degrees above ambient. The heat mats are set to come on at 60 and turn off at 75. With the current temps forecast, they'll be able to keep up with that.
Got my grow light hung up and wired to the controller/timer. It was the brightest 4 foot LED shop light available from Menard's several years ago. They've changed or renamed brands a few times since but still have an equivalent. With the difusser removed exposing the LEDs, it's very bright. I need a couple more.
They all do this so you can plug them together 
Many also have the cord coming out the other end. Both things prevent you from butting them up to each other which I need to do. I built an end to end shelf down the 12' side of my shed that will fit six 1020 trays. Shed is about 136" inside and three 44" lights is 132". The light I have has cord out the end which will be in the way. One I'm looking at from Menard's has cord out the top but the above pic is that model, outlet on end. I'll figure out how to have everything out the top some way, without cutting/drilling into the LED strip modules. Might not be often that I need them all at the same height, butted end to end, but it will happen. Short lightweight ext cords cut in half. Drill small holes in fixture tops. Hard wire cord ends inside fixtures, male on one end, female on other. Also need a relay on each one.
Thinking about a trip to Menard's tomorrow. Couple of shop lights, short extension cords and some tunnel bracing stuff.
Wire rope and four chain link top rails. Will remove all endways movement which should be done before putting poly on ends. Will still need five more top rails in the bigger diameter for roll ups to roll up on. Also need some uprights on the ends about 18" in from the sides. Not sure what those will be but the bottom will go into the ground. Top will attach to bottom truss chords. Bottom truss chords are a little ways up the outer arches, hence they're not full width of the tunnel. In ground uprights will attach about where they meet.
That leaves me with about a 19' wide opening on each end. Works out because top rails are 21' and full tunnel width is a little over 22'. The 19' opening will give the roll up pipes something to hit against. Might need center uprights too, else the wind would probably bend the roll up pipe. Might need two, one on the inside, one outside. Same with the ends. Sandwich the roll ups so they can't blow in or out. Same with the side roll ups but I already have seven inner uprights on the sides.
What's another few hundred dollars of pipe? Speaking of; few hundred in roll up motors. Few hundred for wiring components. Few hundred for new irrigation items including soil moisture sensors. Only a hundred to fix the rocketbombplane wind sensor. Maybe another hundred to mount that and the rain gauge properly. I'm sure I'm forgetting some things but probably only be a few hundred more.
I might go get a truck load of compost tomorrow too. Opposite direction of I-44 from Menard's but only by 15 miles. Good half a day total but Menard's opens at 6am so I can leave a little after 5am. Make it home by 10am.
Hey it's up to 40 and the clouds are dispersing and no wind. Think I'll go look at the tunnel. Make sure I'm not forgetting a few hundred dollars. No wind so I'll burn my dicey spots down by the shed and shop today.