How do you get accurate measurements from your computer? By accepting what "accurate" can mean in this setting. As I learned in 8th grade science class, you're only as accurate as your least accurate measurement. So, seeing as the precision one is likely to acquire from a roll-out test or any other test is probably has an error of ±2cm, I'm now happy using the value in a given computer's table. For example, on my road bike, with the SigmaSport 1600, for a 700x23 tire, the key value is 2133mm and for a 700x25 tire it's 2146mm. That's a difference of 1.3cm. Honestly, I don't expect to get a measurement accurate to 1.3cm so I use 2133 for my computer as I run 700x23 tires.

Now, suppose the measurement is really off by more or less 2cm (upper end of inaccuracy, if you believe my last paragraph). That's < 1% off, so say worst case, 1%. Over the course of a 10000km season (more than most of us will ride) that's an error of 100km, which is maybe one ride's worth if you ride 10k km per year. Do you really care at that point?

Also, temperature, road conditions, etc introduce variations from ride to ride, so one value is never going to be accurate for every ride. Heck, during the course of a ride your weight can very termendously (body's water loss and empty bottles at end of ride), and at the end of a long ride, your tires have possibly lost pressure, etc. Accuracy, schmaccuracy!