the other part, spokes are "normally" measured from the end of the spoke to the inside of the shoulder (DT).
Of course they could also be measured to the centre of the head point, this probably accounts for you see in older spoke tables <?>
But as long as the spoke is in the nipple to near the nipple seat bed (so probably no thread showing inside the wheel) and not fully wound in (you cannot tension it any more), then all will work
Hence the reason you may find real wheels have the same spoke lengths, they'll just end at different points (~2mm difference) in the nipple, you should see that visible at ever other spoke.
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EDIT see DT spokes herehttp://www.dtswiss.com/Resources/Suppor ... pokes-2013
Here's nipple talk (as it has the nice picture of DT's )http://forums.mtbr.com/wheels-tires/nip ... 16327.html
You must remember I'm just a home enthusiast, not a paid professional