You all can mock me if you have already figured this one out, but it has taken me a number of times trying to get a good measurement of something round, like a barrel or trashcan, to come up with it. I would normally lay a tape measure across the top in a number of places, trying to be on the diameter as best as I could, and then average the results. The other night, I was measuring the Brute trashcan I'm building my system around, and was getting diameter measurements that varied by as much as an inch. So I decided to measure the circumference of the can, and then used math to determine the diameter of a circle with that circumference. I haven't built the part that needs to fit the can, but I have high hopes for the accuracy of this method.
Do any of you have any tips like that that you might take for granted, that you wold want to add?