Ordering/Building CV Driveshafts

When ordering a shaft they want the length from the center of U-joint to the center of U-joint while the jeep's suspension is loaded. How do they know from that how long to make the slip joint. Do they use a standard rule of thumb? (ie. 1" compression under load, and 2" expansion under droop?). Just curious.

I'm about to order a new CV shaft and measured with the Jeep sitting under normal load. Then I did some math to figure out what the shaft would need to be when the bumpstops are hitting, and what the shaft would be under full droop. From there I came up with:

Compression - 32.0"
Normal Load - 32.75" (the number the driveline shop wants)
Droop - 34.7"

They usually just put the slip mid stroke. I believe there is about 6" on a standard aftermarket shaft. They usually question whether you're running anything out of the ordinary or something like a shackle reversal on a leaf sprung jeep.