If you're folding the letter in 3 to fit inside a DL envelope and you are using a window envelope the only thing you need to consider is the placement of the address.

The method I have used is remove the film from the window, place a folded blank sheet inside the envelope, and mark the edges of the window on the paper. By measuring the distance from the top and left of the sheet you can then find where the address needs to be placed.

If you are not using a window envelope, then it really doesn't matter where you place the text elements.