I would like to get new tires and wheels for my trailer, but a little confused as to how to measure the bolt pattern. I tried to measure from the edge of one lug to the end of the other (according to picture). This may sound stupid, but does wheel have to be off to make an accurate measurement, or am I doing it wrong. The hub is in the way, right. I would like to order online, so I need this measurement exact. I want the right tires the first time! Any tips?