I’ve dug up a few UK based resources on the Sigma calibration. Seems an interesting one this. Try:
https://www.strangely.org/owned/bikecomp/ This guy fitted one to a Lotus Seven replica and then calibrated it by testing on a measured mile.
https://www.st1100.org/farkles/sigma.html Shows an ST1100 fitting.
If there’s not the info on those sites, maybe an email to Sigma could help via https://www.sigmasport.com/index_e.html