Only if one makes some basic assumptions. One could devise scenarios to ensure more equitable respite.
No assumptions made, well not by me.
The 75% derives from HAL's published 3-runway operating pattern, the stated aim of which is to provide "respite distributed on as equitable a basis as possible".
http://www.heathrowairport.com/stati...ptions_LHR.pdf (Page 19)
I'd be interested to see examples of the alternative alternation(!) schemas that you refer to.