I have mostly selected the head tube angle on my frames based on what I have picked up as being the norm for the intended purpose of the bike. Usually this is within a very narrow range, and sometimes influenced by the angles of available lugs. I have to admit I have not been able to discern much difference in handling between any of the bikes I have ridden and I assume one simply quickly adapts to any differences in handling.
In my wisdom I decided that it might be a good idea to build a frame whose head angle (and hence trail) I can adjust, and I have chosen, taking in design constraints, 55 to 83 degrees, which should take me either side of ludicrous. Furthermore I want the frame to fit wheel sizes of 20 inches, 26 inches and 700c and accommodate up to 57mm tyres so I can test handling of most wheel sizes. The rear dropout spacing will have to be variable from 120 to 135mm and the bottom bracket height will have to be adjustable. Of course it may not work!