The discussion was about using it for a mainsail halyard, so you should be using the P dimension which is 35'. Of course you need the tail, but this is very boat dependent, ie where the clutches are, how things are run. For this boat that would mean 70' plus tail.
The length of the headsail halyard is =square root of (I^2+j^2)+I. For the Alberg 35 this would be 42+40=82' plus whatever tail you want.
You can certainly go with a larger size, but I can't see a point to it. The SK-75 has roughly the same stretch as wire, and while there is some misconception on the Internet about this, there is no appreciable creep in dyneema line
when used for halyards. Creep is measured over the course of years, not hours or even days, and takes significant (as a percentage of breaking strength) long term load to even be an issue.
The major issue people see with stretch in dyneema lines
is actually their sails stretching, or the line
slipping through clutches (this is why the covers are so important), not actual elongation of the line