But isn't that 15% already added to the GOOD, MEDIUM and POOR, PI QRH data?
You are quite correct, my mistake. It states, for 'normal operations' that:
'All reference distances and adjustments show have been increased by 15%'
Contamination may well degrade the braking action further due to aquaplaning, slush or snow build up under the tires etc. Therefore the tables will give you ALD then factor 15% for safety.
At the end of the day it is a judgement call as no amount of tables could cover the infinite number of variables that mother nature throws at us.
As for choosing exits it is just as you say, look at the landing distance to the exit you want and then calculate what configuration is going to give it to you 'comfortably'.