1. Axle position: Just as you should when doing a link suspension, build at full bump. To do this with leaves, go ahead and disassemble the pack down to just the main leaf. Position your axle where you want it to rest at full bump.
2. Fixed hanger location: With your axle located at bump, the location of your fixed hanger will be equal to the distance from you center pin to the center of the spring eye ALONG THE ARCH OF THE SPRING. Front or rear axle, forward or rear shackle, it's all the same.
3. Shackle orientation: In the case of a front axle, there are some further considerations as to which end of the spring you want to fix. The most popular method is to fix the front end of the spring. Popular thought is that this allows the axle to move backwards when it encounters bumps, yielding a better ride. For offroad applications this presents a few problems; under droop the axle will move away from transfer case. This requires a long-travel driveline. Slip lengths of 12"+ are not uncommon for the front axle of a leaf sprung rig. Long slip drivelines can be expensive, sloppy, and heavy. Another problem occurs when climbing. Under power, the axle will drive itself out from under the vehicle, unloading the suspension. This is counter-productive to the task of trying to climb as the axle will not pull any weight until the suspension limit has been reached. This can also require even longer drivelines to keep up with increased slip requirements while the axle is under power. A third problem occurs under compression situations. Under compression the axle moves rearward and towards the vehicle/ body.
This can make building a low-slung rig even more challenging as it is possible for a tire to recede several inches towards and into the body during compression. Depending on your desired tire size, it may not be practical to move the axle far enough forward before you run out of frame to attach your spring hanger to. Earlier I spoke of popular thought concerning rear shackles on a front axle. I disagree with the blanket statement that a rear shackle rides better than a forward shackle. While wheel recession should not be discounted, a forward shackle suspension pivots from a rearward fixed location; just as nearly all front link suspensions do. These have been proven to be plenty fast, when setup properly.
Forward shackle suspensions do not require a long slip driveline because the axle is pivoting from a fixed end near the transfer case (same as a link suspension). During climbing, the front axle will constantly be pulling forward, on the fixed end of the spring. It is also easier to build a low slung vehicle with a forward shackle because under compression, the axle will move away from the body. Yet another consideration in this particular debate deals with geometry. I once stumbled upon a reference that suggested that the shackled spring eye, should be located higher than the fixed eye, relative to the ground. I have not been able to track it down the last f year, but consider OEM applications. Nearly all are setup this way, on both front and rear suspensions (with the exception of Toyota mini truck front axles). It is much more easy and practical to achieve this geometry with a forward shackle setup on a front suspension.