I did a test run on this save now: http://www.matrixgames.com/forums/fb.asp?m=4997654 . Using B1147.20.
The RVs at the edge of the nominal THAAD range were a lost cause (too late to intercept), so I spawned a fresh DF-26C TEL in the vicinity of the original shooters and fired another shot.
The missile & RV were detected by the SBX platform at over 900nm, and the first pair of THAADs were launched well outside the nominal envelope:
Do you mean automatic launches or that you were able to launch manually?
I re-ran the scenario from the beginning to ensure all behaviour was within B1147.20 (launch happens early on), and saw that the targetting stance goes through several phases:
Phase 1 - about 940nm out, shortly after the RVs are detected, I found I could manually target them all.
Phase 2 - about 540nm out, when the salvo enters the THAAD FCR zone, I can still target some but for others "Target is outside the weapon's DLZ zone".
Phase 3 - about 450nm out, some are "Target is outside the weapon's DLZ zone" and others are "Interception point is below weapon minimum target altitude"
Phase 4 - about 420nm out, all can be manually targetted again.
Phase 5 - about 220nm out, as the salvo approaches THAAD missile range, all RVs are "Interception point is below weapon minimum target altitude" and remain that way until impact.
I got no automatic launches to occur, using default doctrine.
Interesting that the THAAD C-2's horizontal range is somewhat less than the vertical ceiling - in practice I'd expect it's more complex, involving total delta-V, so that lower intercepts could be achieved further out. Though of course the more of the trajectory within the atmosphere the less delta-V can be achieved due to drag.