Maybe I'm not reading you right but you say using perfect technique and approach you would stop at 60%.
Technique is irrelevant, as is the actual stopping distance.
The 60% requirement applies to planning the flight and to the dispatch; it does not apply to the actual landing, nor does it set a any requirement that the aircraft actually stop at the time of arrival within 60% of the available landing distance.
These are two separate issues. One involves planning; if one cannot establish in calculations using approved data that one can stop within 60% of the total landing distance, then one cannot undertake the flight to that runway under those conditions. Something must change, or one must go elsewhere, or one cannot go.
On arrival, it's entirely different. If one exceeds 60% of the available landing distance in actual practice when one lands, one has NOT violated the regulation. The regulation applies only to planning the flight, and not to it's execution.
When you speak of a "factored runway," you're thinking of a fully factored runway that's wet or contaminated. In that case, you must add 15% to your total stopping distance, and that "factored" distance (stopping distance plus 15%) must then fit within the 60% limit of the total stopping distance available.