I use a bit of pressure on the tube to bubble it up a bit (maybe an 1/8") and swipe it off with my finger. I just put a thin coating on the tapered barrel and turned it a few times. Then check to see if any is blocking the openings. That would be an indicator that too much was applied. Since it is gas-resistant, it would interfere with gas flow. BTW, it also works on petcocks. It stops the black goo that has a tendency to drip down from the end of the tapered barrel there.
With the amount that you use and the size of the tube, now do you understand why I said it would last 5-10 lifetimes? :-)