Replying to True Center = Tip-to-Tip/2 or Running Surface/2 ???

So I'm getting my scratch fs wrs's mounted -2cm from center and want to clear something up...

I just read a thread - and I can't find it - that said if you're measuring based on the true center point, you need to measure the ski's running surface and divide that in half. More people agreed with this than those who said measure from tip to tip of the ski and divide in half.

The argument was that if you measure tip to tip and mounted true center - you actually have more ski behind your boots than in front (because of the lift of the tips off the ground), not what you normally want. But if you measure the running surface, you'll actually be skiing true center based on what is touching the hill while riding - which I think makes more sense, right?

So which is it? Anyone have a solid answer?