You shouldn't need a schematic to calibrate the vertical channel. Just feed it a fixed voltage from a high precision voltage reference, should only cost a few bucks to get 1% or better accuracy on a fixed point, opamp multipliers and divider networks can derive any voltage you want from the references for a multipoint refrences to determine linearity, but that might get a little more expensive.
I check mine against a 5volt fixed regulator with a light load.