The weight was that which allowed a perfect no problems takeoff just before the runway ran out .... barely. I always wondered how reliable that calculation was
An opportunity to tell my favorite KC-135 story.
When the 198th at San Juan converted to A-7s from zippers, the Tucson Guard offered to provide the PRANG with combined Departure Training and DACT upgrades. They liked the idea and we set up two F-100s to be tanked non-stop to San Juan from Tucson with two A-7 IPS on board the 135 (incl myself) to do the A-7 Departure Training.
KC-135 takeoff was scheduled early morning out of D-M when it was cool, with 2 clean Huns out of Tucson IAP next door launching at the same time.
Well the AFRES 135 out of Mather developed some radio problems and the launch was delayed. Ambient temperature went up about 20 degrees before the problem was fixed. 135 took the runway at D-M and then commenced to burn down fuel for about 10 minutes with the brakes applied. Coordinated with the Huns over at TUS and when their calculations were met, everybody released brakes together. I didn't have a window seat, but other A-7 FWS guy did. As we rolled down the runway, he counted the 1000' remaining markers.....9,8,7,6,5 etc, etc. When his eyes got big I knew this was going to be close. He got to 1 and then I felt us roll over the departure end cable and we got airborne in the overrun.
We didn't get higher than about 300' going thru the pass for I-10 between the two mountain ranges to the southeast. Easy join-up for the Huns and finally off we went to San Juan....evidently a reliable if somewhat cavalier calculation by the 135 dudes. Whew. No nukes in the vicinity.
But then we finally get near San Juan, with CBs all around and they offload some extra gas to the Huns just in case. We land minimum fuel (or less) and the mech or somebody comes back and says, 'everyone forward so we don't fall back on the tail until we get the tail stand in place'. Whew again.
After a week in San Juan, the same Mather AFRES 135 crew shows up to get us back to Tucson.

Fortunately going west we had an overnight stop in Miami first.
I've been in the last 1000' on takeoff in the F-100 and the USMS 727, but never on the overrun....I like our calculations better.