How much would a low tunnel extend my season on the front end and should I start my seeds sooner ?

I know that seeds are usually started around 6 weeks before the last frost ( if indeed you are starting them indoors ).  I was looking at the average frost times in my area fronttimes and it looks like June 1 is the last frost, which means that I would get my seeds started around the second week in April or so. I'm planning on using plastic mulch for the first year, just to get a head start since I'm going from grasses, and I don't want to till. I'm also planning on building low tunnels over the beds. I'm wondering, with dark plastic mulch, and 6 mil low tunnels, how long am I looking at extending my season by? does that mean I should start my seeds earlier?

They say a hoop house gets you a zone and if you have a cold frame inside a hoop house you get two zones.  I think it's Elliot Coleman who says you get a zone for every layer of coverage.   You can also winter plant seeds in a container, something like a milk jug or a pop bottle.