Its right there in the FARs.... How you calculate you distance to 35 ft is based on 115% of actual. Its buried in the AFM, but it is there. So no matter what runway you are on, just from the FARs alone there is 15% of buffer or slop. On 8100 ft runway that is 1215 ft or if everything is as it should the aircraft should be at 35 ft around 1200 ft before what you calculated said it should.