Something everyone seems to be missing is whats going on above the frame... the overall steering response is going to be affected by the position of the bars in relation to both the steering axis and the axle of the wheel...
Part of the reason head angles have been getting slacker is due to shorter stems (which quicken the steering to counter the slacker head angle) becoming more viable due to wider bars (which provide increased leverage, whilst the increased turning radius provides more controlled feedback)
Looking at BMX and Brompton, the elevation and offset are obviously going to contribute to the overall system, but there is so many variables, my mortal bonce is beginning to melt... I think so much of current geometry has come about by trial and error, rather than a tangible mathematical equation.
You may be able to calculate the physics of the aforementioned 40ft double, but to hit it is the only way to prove the equation!!