You've got an awful lot of tall plants - how are those guys in the middle going to get any sun? :) Generally, you want to position plants shortest in the front to tallest in the back - SFG puts most vine-like plants on a trellis along the edge. With sun-hogging toms on both sides, you've created a valley of shade that will keep your peppers and cukes from growing.

As for when to start seedlings, just work backwards 8 weeks (for toms - 2 weeks for cukes, 6-8 for peppers) from your planting date. For a planting date, work forwards about 2 weeks from your last frost date (for these heat-loving plants). As for your last frost date, well, that's pretty much a gamble! Check weather sites and see what it's been the last few years, stick your finger in the air, and make a guess.

I didn't think about light. I'm glad you caught that. I will rearrange my plan. Thank you!

Mary,
I think the bed will be really really full (it is a 4x4, right?). Check out the link below. You can see what happened to mine...

Here is a link that might be useful: Plan vs Reality

Seems like you've got a lot of tall things on the edges and may not be able to reach to the middle even if it's just two feet away.

You should be giving zucchini 3x3'(9 sq ft) of growing space, one would take up most of a 4x4' bed. You can plant 2 cukes to a sq ft. If you prune your tomatoes to just 1-2 main stems you could grow them 1 to a sq ft. Peppers are good at 1 to a sq ft. I don't know about the spacing on melons, but I think they take quite a bit of space.

I think you may want to rethink what you want to plant in that 4x4' bed.

It's important that you are able to reach the center of the bed without stepping or leaning on the growing soil. I make my beds only 3' wide, my reach is not as long as some.

You may want to consider growing that melon in a container, I have read alot about people who grow them in half barrels or even trash cans. The container garden forum may have some postings about that.

Good gardening, Mary