You won't get (or shouldn't) a specific length. You need the shop that installed the lift to set the castor castor correctly.

 

juz sayin, low caster doesn't cause DW. Sure, 3.5* is a touch low, probably a full roation on the upper arms, but if you got wobble woes, you got more issues. I'd bet undertorqued TB bolts given the lift was just put on.

 

It is deathwobble, and not some form of wandering/bumpsteer/shimmy? If so, you are going to need more help than just adjusting the ca's, and might want to start your own DW thread instead of tacking it on here. Be sure to post the rest of the alignment specs as well as all of the components in the lift (steering correction, trackbar brackets, etc), better detail on what specifically you feel (violent shaking and you need to come to a stop, general wandering side to side, quick pull/jerk to one side, etc.) and what triggers the steering issue if it is a certain event (driving over one particular seam or bump in a road, or something like that).