Landing
CHR
Why do you say that using FM landing distances is not a good idea? They are perfectly fine to use, that is why they are published. When the aircraft is test flown they measure the distances used and factor them for the average pilot in an average aircraft on the average day. If you come over the threshold at 50' or less at Vref with the pwr at idle, like you should on every landing, the distances are more than adequate.
The CAT factors are to cater for people landing beyond the ILS and being too fast.
In the good old days there was a factor to use for your destination and much lower factors for your alternates based on the fact that pilots could become complacent at regular destinations but when they divert they would be concentrating and flying more accurately.
The book figures are perfectly good if you fly accurately.
But to answer your original question you must use your book figures but check the text about whether the figures are already factored, as many are.
MM