The 'regulations'stipulate that aprox'(57%)distance from VR to screen height must be over Runway.This is where'clearway'comes into play.This is the third distance required after Accl/stp..Accl/go..All engines Distance +15%
It becomes a concern when JAA allow you to use'clearway'(500/1000')to get to your'wet'15'screen-one crosses the end at about 10'......thoughtful eh