I think everyone accepts that the intention of mandating turbofan aircraft to land within 60% of the available runway is to increase the safety margin but its a bit strange that a linear factor is used to factor a quantity that varies with the square of the speed. The way the regulations are currently applied, the actual margin of safety is greatest when you are light and at a minimum when you are heavy. If the amount of brake energy you can exert is reduced by the most common constraint, friction (a reduction in mu), that's a linear reduction. Which means the amount of braking you get decreases in a linear fashion whereas the amount of energy you need to dissipate increases with the square of the speed.
It would be far more sensible if the same factor of safety were applied. More complicated maths but in the days of APG and flugP, who cares. It would mean you could carry slightly more fuel or land on that slightly shorter but very useful runway.
Here's a question. Is it 70% for turboprops because the effect of beta is allowed? And if so, given the failure rate of thrust reversers, will we be able to use data which takes TRs into account at some point in the future?