Are the corrections for landing distance linear?
Energy to be dissipated increases with the square of the speed don't forget. I would suggest that using the QRH Normal landing distance figures to extrapolate to Vref40+70 would not give you an accurate answer, and you have know knowledge in which direction the error is.
I would class that as "brave" behaviour personally.
"So, Bloggs, with the QRH non-normal distance there why did you invent your own landing distance calculations?"