If you can see the milky way at your place I would consider yourself very fortunate! I went observing at a dark sky site a couple weeks ago (N Central FL) and the milky way was the best I had seen in 25 years!! If you are intent on building a scope yourself, my recommendation would be an 8" in the f/7 ~ f/9 range. f/7 would be a little easier to figure as the parabolodial shadows would be a little easier to see and measure accurately. At f/9, about time you see the shadows distinctly you are almost overcorrected, but not quite. The f/9 would fit in a tube that would place the eyepiece at a little above eye level unless you are 5' 18" or therabouts! My first scope that I built was an 8" f/7.5 and it was a beaut. Good for deep sky and also for planets. You have to remember, MOST of the time the skies aren't going to allow 400x no matter how good the optics. So don't design the scope with that as a primary consideration.