OP, here is a pic of the lean-to I recently built off of my shop:
Attached FileThe lean-to is about 12' off the building. The beams are twin 2x10's. The posts are 10' on center 2 7/8" oilfield pipe. For the posts, we drove 8'ers into the ground pretty well as far as the driver would drive them, then welded a 2' long "male" 2 3/8" pipe into the top (1' in the 2 7/8" pipe and 1' sticking out). When that was done we shot all the 2 7/8" pipe tops with a transit, recorded the measurements, figured out what the height of the beam would need to be, did some math, custom cut each top pipe to length, welded a heavy piece of angle iron onto the top (to bolt the 2 x 10 beams to), then set them onto the driven posts.
ETA: you may notice some extra splices in the pipe posts in my crappy grainy iPotato pic. This is due to me making the notes from the transit measurements, then a buddy doing the math on what the top pipe lengths should be... We weren't quite on the same page, and as result I had to make some corrections.
ETA 2.0: The posts are plumb (or really close to plumb). I think they look all wanky and out of plumb because of the angle I took the pic from.