The belt's important. Make sure your jacket waist size is larger--how much is hard to say--than that jeans+belt measurement so that you're not squeezing yourself when u want to zip up. Maybe the old "your measurement times 1.1" p2p math would work here?
For shoulder measurement, feel the top of your shoulder. Do you feel where the bone stops? Measure from there to the same spot on the other shoulder. Of course if you've got a jacket that fits you well, I'd trust that measurement too. I was gonna say that 16.5 sounds like its on the small side for shoulders at 5'7" 150--not too small necessarily, but smallish, and everyone is a little different anyway.