1 mile on 85 seems pretty reasonable to me. Why do you need it any more accurate than that?

To calibrate it properly, you'd have to mark out a course using a certified measuring device (like what the cops use to measure skids etc) and then ride your bike along it. Exactly along it. Probably best to average the results from several rides. The longer the course you mark out, the more accurate it will be.

Once you get the computer-measured number and you compare it to the actual distance, you can adjust your wheel diameter settings on your computer to match.

So, for example if your "rollout distance" on your computer was set for 2150mm, and you're out by one mile on 85 you'd need to increase the 2150mm to 2175 to correct it.