beisdes, if you're only 18 you've probably got some more natural growth yet to come. You might possibly get some great growth over the next couple years. Be patient my friend and let nature do it's thing. You are already well above average so be happy.
I believe ya man. It can be difficult to sustain a hard-on while your concentrating on positioning the ruler just so. Also hand size is not really an indicator of size, because there's a lot of variation between people. If you really want to convince people, you should use a standardized product, like an iPod or a water bottle. I would also suggest that you use a webcam so you don't have to fumble with a camera/phone. Try to get the entire ruler in the shot too.
As for your other comment about making your dick bigger, I'm curious about why you want to. I'm doing some jelqing routines and their at least making my erections stronger. But for me it's more of a curiosity thing.
Apparently, measuring your girth with a ruler does not give you an accurate number. S0, according to the sewing tape I'm 8.5"x5.5"