p/v clearance threory-- bogus or not? what do you think?

[DRASTiK]

No discouragement here. Thanks for the reasoning. I figured that little formula seemed a little too simple to be accurate. That's why I made the thread.

I've got an air compressor and I know that you can use compressed air to keep the valve from opening while compressing the spring.
My concern is how exactly to go about doing this. Do I fill it with air, then stop the air without releasing the air of course, then compress the spring and hope the valve stays closed or do I keep the air going while I compress the spring? I was trying this earlier, but the valve kept opening and the air would leak out the ports to the intake.
I also tried using nylon rope to hold the valve closed, but had no luck there either. Maybe I wasn't trying hard enough for fear of mangling a valve. I've got weak springs to use while checking the clearance, and a spring compressor, but they do me no good if I can't get the current springs off the head. Any thoughts?