First a question that I have asked before: When you punch a hole in the lip, isn't there a chance a spark would cause a BIG problem?
I'm not sure about the ventures but on my RSTD, here is how it works. When the guage goes to "F", the countdown begins, and when the engine starts sputtering, I switch it over to reserve.
Now if I'm riding normally that will be at around 15 miles on "F", (before I switch it to reserve) then I can go another probably 30 miles, but like when I was out west, (pulling a trailer) sometimes it would be at 7 to 8 miles, then I'd say I can go about 15 miles. Mileage was 26/27mpg then, so I judge how far I can go by how many miles on "F" before I switch it to reserve.
So the wind plays the biggest part in this for me. And I have a smaller tank to boot.