Forget about the bike for right now.

Get someone to help you taking your measurements. A roommate, sibling, parent, friend, whatever... It really does take two people to do properly. Do it like the videos outline. Take them 3 times and average the results. Try and be a little more accurate, bodies aren't normally in exact inches. Get your values to the nearest sixteenth of an inch.

Take an hour and do it right, or there is really no use in doing it.
It's a calculator. Just like any calculator, if you put bogus numbers in, it won't magically spit out the correct ones.