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.

You might want to actually explain that in your original post then Penner, I think the distinction is kind of important considering you said you needed to calibrate a vertical channel, not that one was faulty and you were trying to trouble shoot it's failure. That has nothing whatsoever to do with calibration.

Some scopes have a 10X selector knob perhaps your should start there since your failing vertical channel is off by nearly that much, perhaps it's stuck on.

I have to agree. If it was out by 10% it would be a calibration issue. 10x is a faulty scope or a faulty operator.
The ~10x control is also called VAR on a lot of scopes and, on Tektronix scopes at least, is a 2nd concentric knob on the Volts/Div knob so it is easy to turn by accident if you are ham fisted. Usually there is a warning light to tell you that it is active.

Perhaps the 10X works fine and the LED is broken =\

No problem, biggest mistake is not enough information. Believe me, you can't bore us with too many details.

One thing you could do is feed an identical signal into both channels and compare the reading between them at various points. This will only really work for you if the channels are laid out the same as one another. Another scope would be best for this but you may be able to use a voltmeter if it's input impedance is 10MΩ or higher.