I calibrated mine using a GPS app on my iPhone. I taped the phone between the gauges so I could easily see it and zeroed the gps mileage and activated the speedo calibration while stopped. I then rode down the straight road at around 40 MPH and slowed down at 9/10ths/stopped as the GPS hit 1 mile and pushed the speedo cal button.
Seems quite accurate - measured using the same GPS app and also based on the OEM gearing/sprocket/RPM/MPH info in the service manual.
FWIW - I almost removed the E-speedo after installing it, becoming quite frustrated with the calibration, especially since my OEM speedo worked fine! I found it rather difficult, frankly, the button pushing - or my failure to do it properly - caused me to have to re do the measured mile several times before it actually "took." I tried the static method first and got so PO'd doing that that I decided to quit for the day before taking a 3 pound sledge to the e-speedo!
TBH, as far as a recommendation, If the stock gauges work properly, as mine did, I don't see any worthwhile benefit. I bought them primarily for the speedo, with the intent of eliminating the speedo drive gear. I have never have a problem with it but while on the IOM last year, there was a guy in a pub parking lot dissembling the rear wheel of his Commando because the drive gear had become red-hot and seized. I thought...hmmm, might be good to get rid of that!
BUT I still have my OEM drive gear because when I bought the gauges, the supplier did not have any of the separate rear wheel sensors so I used what came with the E-Speedo, which plugs into the stock speedo drive gear. So basically, I spent the money on E-gauges to replace perfectly functional stock gauges and STILL have the Speedo drive gear.