I must agree with tgg, I work for a calibration company in the NE England, we calibrate everything from dmm's to complex PAT testers, scales, you name it. We have calibration equipment that costs tens of thousands of pounds, all UKAS approved.
It depends on whether you use your scope for hobby or professional purposes. If hobbiist, and your function gen and DMM is more accurate than your scopes spec then you can use those, if professional, then it should go to an approved calibration company.
As tgg mentioned frequency, you need to test at low and high frequencies. Dave has lots of videos on YT on oscilloscopes, watch them, fantastic info, you can't get better.