RNWT -
(Job 42:16) "After this Job lived for 140 years, and he saw his children and his grandchildren—four generations."
I liked arithmetic at school; 140 divided by 4 = 35!
Also -
(Numbers 32:13) "So Jehovah’s anger blazed against Israel and he made them wander about in the wilderness for 40 years, until all the generation that was doing evil in the eyes of Jehovah came to its end."
It appears that the RNWT indicates a generation to be 35-40 years. On that basis, the WTBTS's "overlapping generation" should have terminated no later than 1980 - at a stretch!