I'll take a swing at this. Let's say the axle is laying on the ground in front of you and you are standing over it looking down at the "smile". Extend a line from the centerline of one spindle to the centerline of the other spindle using fishing line or string or whatever. Transfer that line to the ground, making a mark all the way across between the two spindles. Measure the distance from the mark you made from using the line to the upper surface of the axle where the spring perches would mount. Now, measure the vertical height of the axle itself. Add half the vertical height of the axle to the measurement you got from the spindle line to the spring perch. Let's say the "smile" distance is 100 mm. Let's say the vertical height of the axle itself is 60 mm, half of which is 30 mm. I'd say in this case, the drop would be worth about 130 mm.