Don, I agree, I agree,
As for actually pointing to an elevation and saying the NPP is about here, somewhere in one of these posts I explained how to do that and it is rather easy. All that is necessary is a ▲t and a manometer. From inside and outside temperatures, a stack pressure per foot can be calculated. Then, by taking a high or low pressure reading (that's where the pressure will be the largest) through the envelope, you have it. If the front door reading is 2pa and you had calculated a 0.25pa per foot stack pressure, then the NPP is 8' above the penetration at the front door.
If you want to get fancy and you have a good ▲t, you can open a window a measured amount and, just like the "add a hole" process for pressure diagnostics, you can estimate the CFM50. I keep baiting people with this in hopes that someone will do the math for me :). My to-do list it too long.
As for the metal capillary tube, you can't get too small as a manometer does require a flow of air. I would have to ask to be sure. Just an FYI.
PS, I have started another thread to get into this issue a bit more: http://homeenergypros.lbl.gov/forum/topics/controlling-passive-air-...