Sounds like you have had some fun working with your computer!

I have always rolled out the circumference for my computers as well... although there are several different ways to do it, here's what I do..
My computers have always measured the same as the odometer on my car...

Start with your valve at 6:00 and make mark on the floor from that spot... roll the bike in a straight line until the valve is at 6:00 again, and put another mark on the floor. Take a tape measure and measure the distance between the two marks... if you don't have a metric tape measure, you can always use the 2.54cm= 1 inch.

I also would mention to you that different brands of tires(even if they are the same size)measure differently, so make sure that you measure them to be accurate.