I forgot the most important bit....
Having done the maths I like hfd's 85% at 40% - so that's what I'll use.
With respect to 2/3 at 1/3 that appears to be much closer to the line (if not on it).
As far as Mass & Power go it doesn't make any difference to the shape when the graph is normalised to 100% of takeoff speed at 100% distance allowed.
Finally, the comment about obstacle clearance, is well taken - my observations above only apply to getting to a sensible speed at the end of the take-off run.
OC619