Short answer is actual earnings. Because its based on actual earnings prior to going on LTD a future hypothetical pay increase doesn’t matter.

“A pilot shall be eligible for LTD Plan benefits upon exhaustion of his regular and disability sick accounts, as described in Section 14, or upon his experiencing a seat change or upon moving to a non-pilot position because of disability, as described in the LTD Plan . Except as provided in Section 27.J.7., disability benefits paid to pilots who are disabled prior to October 30, 2006, shall be governed by the terms of the disability plans in effect on the date of commencement of the dis- ability . Pilots who become disabled on or after October 30, 2006, shall be governed by the provisions below .
LTD Plan benefits payable to a pilot whose disability commences on or after October 30, 2006, shall equal 60% of monthly earnings, as de- scribed in Section 27.J.6., for the first 24 months that a pilot is eligible to receive benefits under the LTD Plan, subject to the limits in Section 27.J.7. Thereafter, benefits payable to a pilot who becomes disabled and continues to have an occupational disability shall equal 50% of monthly earnings, as described in Section 27 .J .6 ., and subject to the limits in Section 27.J.7. Such benefits shall be paid through the earlier of the date on which such pilot (1) ceases to be disabled, or (2) the date on which the pilot attains age 65 .
In order for a pilot to continue to be eligible for a disability benefit under the LTD Plan, the pilot must fully cooperate with the Claims Paying Administrator, in coordination with the Aeromedical Advisor, and must diligently seek restoration of any required license or medical certificate to allow the pilot to return to work.
The Company shall pay the full cost of such coverage .
Monthly earnings shall continue to be defined as a pilot’s basic monthly compensation, i .e ., the 12 highest consecutive months out of the 36 consecutive months preceding the disability period .”