Shouldn't ATC be aware that not the true but indicated mach number is being reported via ADS-C?
(Maybe there even is a correction implemented in the ADS-C module for your airplane, you could of course just ask ATC what mach speed is displayed for them)
No matter what is reported to ATC you should follow your clearance, shouldn't you?
Anyway maybe one of those two links can help you:
Mach Number Technique
https://www.faa.gov/about/office_org...anges_2017.pdf