What is the typical angle between the derailleur post (seat tube) and top of chain?
There is not a fixed angle between the front derailleur and the chain, because the angle of the chain relative to the bike changes depending in what front and rear sprocket it's on.     

Chain stay angle range is a parameter specified for front derailleurs. As an example, see this Shimano specifications page for road groupsets. Chain stay angle is the angle between a line drawn from the bottom bracket axle center and the rear wheel axle center, and the seat tube (or a line from the BB axle center to wherever the front derailleur is mounted if there is no traditional seat tube).

If you place your front derailleur on a line at that makes an angle to the chain stay line in the specified range, you know the chain will be at an acceptable angle to the cage in any gear combination.