I'm building a (non-standard) bike and that means mounting a front derailleur in relationship to the incoming chain line (rather than selecting a seat-post angle visavis chainstay angle, etc.) So I'm wondering about the typical angle between the chain (on of the crank) and the derailleur mounting post (usually a seat tube). I've measured several bikes here (also non-standard, some standard) with my digital angle finder, chain in the middle of a triple ringset, and I get values from 68 degrees to 75 degrees. All shift fine. I'm wondering if there's a best practice reference here. Most of the seat-post angle stuff I see is in relationship to the chainstays, not the 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.
The answer is rather simpler than I thought. The angle between the mounting tube and the chain (not the chainstays, and not the chainstay angle, since chainstays aren't horizontal, and aren't relevant to the recumbent chainline) is about equal to the seat-post angle on a standard bike, since the top of the chainline on upright bikes is generally about = to the horizontal from which the seat post angle is measured. That means when using a road front derailleur on a bike with a significantly non-standard chainline (e.g. no chainstay angle to measure) then use an angle of about 71 degrees between the post and the chain line. If it's a mountain front derailleur, something slacker to match a mountain bike's seat-post angle.