It's simple, if you raised or lowered a vehicle 4.0" you would want a 4.0" overall longer or shorter shock to keep the shock within the intended design, but you gain or lose 4.0" travel.
If you want to raise/lower 4.0" and keep the same travel, then the shock mounting points must gain/lose 4.0" to keep the shock in it's intended design and the shock stays the same length.
When it's raised with a stock shock you gain 4.0" of compression stroke, lowered you lose 4.0" of stroke, which brings this to where I'm at, I can bottom my shocks out easily, (they are probably already damaged)
With 12.1" total travel, you have 6.05" comp/droop travel, lower/raise 4.0" that would leave only 2.0" available comp/droop depending if lowered/raised, that's if Ford put the ride height in the middle of the shock travel, they may have biased it toward the comp side,
The proper way to shorten a shock is to cut and re-threaad the shaft and to cut the housing down, I have done this many times on race cars, typically Mcpherson strut cars, sealed shock cars you move the mounting points, Raptor rear shocks are similar to struts.
I would lose some travel but I wont be bottoming out the shock, I choose this method over cutting off the shock perches and welding them up higher, that way I can always put back on the stock stuff. (I may do a small shock drop mount on the axle to gain more travel) I will mock it up, but I figure the frame and body might not allow the full 12" travel at a lower ride height without modifications.
Because I don't know the factory installed length I can't do the math on how much to cut, it is factory installed length, minus new installed length, divided, that is the number I need.
I was given 29.5 as a installed number but that's the freelength number not installed.
Example; and I will use random numbers, 25.0" factory installed, minus new installed length 21.0 equals 4.0" divide by two, equals 2.0", shorten it that value and you would put the shock piston right back in the center of it's travel, the same equation holds true for raising it but you would have to lengthen it.
So after this long drawn out explanation and 2 cups of coffee, I hope that is a reasonable breakdown of the how and why I'm doing this.
The part I didn't go into, it's lower so my old wore ass out skeleton can load my 140lb High power ebike (9Kw) in the back without a ramp, as anything in the back of a truck gets stolen (like a ramp while you are out riding) and I'm on my 3rd shoulder surgery, 3rd knee surgery and a back surgery, I really need airbags to drop it down Ha! And NO, a different truck is not a option.
So, if anyone could please give me a eye to eye rear shock measurement on a stock height truck that would be great, I did email Fox requesting the info but haven't heard back.