Try a derate and assumed method.
We have a derate and use the assumed temp reduction method. That is probably why you are seeing aircraft use 70 to 90% of the runway.
It does not detract from the fact that the DECISION SPEED at which you must decide to go or stop. it has nothing to do with how long the runway is as those calculations have already been made.
I have used 4500' on a 6500' runway and 8000' on a 10500' runway the power produced on those occasions was a little different due to the different derate but the principle is the same.