first, if you have an add a leaf your already going in the wrong direction.
Soft, multi leaf springs, proper shackle angle (shackle damn near flat when spring is at max length, calculate this by taking the length of your main ply when flat, minus the length shackle you intend to run, plus 1-1.5"s) & proper shock angles/mounting are what need to be addressed for healthy leaf sprung flex.
Here's a pic of my rig at about half flex with 7) .250" toyota leaves each side, with a healthy shackle angle & lower shock mounts run length wise so the shock rotates on the mount as the suspension moves.
To get anymore flex then that you'd have to get into those crazy drop boxs, incredibly long spring packs, or missing links & such.
But the fact of the matter is, if your locked up front & rear, whether your tires on on the ground or not, as long as your rigs stable & still moving - thats all that matters.
BTW: not that it matters, but I've maxed out a 30 degree ramp with my setup & my rear pack is WAY too stiff right now since I don't have my anti-wrap bar on yet.