I guess after 18 hours with no reply everyone else doesn't know, or are grammatically confused and can't understand the question.
I think the answer to the basic question is measure it to the back (sail side) of the mast, otherwise sail makers would print 2 lots of boom length dimensions, one for SDM and one for RDM.
Use a tape measure, but adjust according to conditions once you are sailing it.

I have another question regarding boom length. To get the correct amount of outhaul, do you pull the sail all the way back so it contacts the clew, or do you just pull it back to the 0 position marked on the boom?

There seems to be a bit of confusion on this.
I have previously been told what easty said above and so that's what I believed, but,..
A few weeks ago I bought a new boom which is supposed to extend out to 222cm.
When I pull it out to the last stop and pin it there, I measured the distance as 222cm from the front of the mast diameter to the inside edge of the clew.
So now I don't know.

Is Albert Einstien on this forum by any chance?
We need someone who is familiar with space/time distortions to answer this one with some authority.
Actually, someone who makes sails would do since Albert is off doing other things.

Maybe someone else can measure their booms in the fully extended postion and see how they measure up compared to the stated extension range, and post it here.

front of mast to inside of the end of the boom.

Brett is spot on. measure from front of the mast where the boom connects to the end of the clew.