Properly applying projected tolerance

Properly applying projected tolerance

I accidentally submitted this as a "Ticket" yesterday, but thought it might be good community knowledge. 

Original question:
I found the article , however, I would still like some clarification on properly applying a projected tolerance.

 Regarding the "Top" and "Bottom" Reference Lengths; do they refer to the "base" of a measured feature? I have a part right now with a projected tolerance of .200" on the OD of this part. One of the features, CMM-M sees the base towards the outside of the feature, the other feature it looks like CMM-M is seeing the base at the bottom(inside) of the feature. I'm wondering how this will affect the number I put in the Reference Length.

QxSoft Response:
The "length" will always be the error over that length regardless of anything such as base position or direction.

 Assume you have a cylinder with a nominal length of 20mm (where the base is or direction is doesn't matter).  You INSPECT 15mm of it.

 If you specify a length of 20, it will take the error of 15mm / 15 (ie: per unit) * 20.  If you specify 50mm it would simply be the per unit (the 15/15) * 50.

 If you specify a cylinder with a base at -100 with a length of 50, but measure an actual cylinder (in space?) at -30 and -10 (20mm worth - but beyond the definition) it will still give you the same answer.