The down tube angle is the result of a number of other frame measurements like HTA, axle to crown height, bottom bracket drop, downtube length, head tube length, etc. It is not a number that you choose, it is simply the result of all of those other frame design choices.

In terms of how much head tube to leave below the down tube, I like to leave at least 20mm. I like the look of it and it keeps me from having to weld to the thickest part of the Paragon head tubes.