One thing you could do without having to calibrate is to compare the spectrum of the antitwilight sky with that in different directions. Just divide the two spectra to see the difference (Adjust the exposure so the intensity is similar to reduce problems with non-linearity of the camera and be sure to switch off any automatic colour correction in the camera).
Note though that some reflective gratings respond differently depending on the polarisation which changes depending on the direction in the sky. This is an example of the response of a reflection grating. S and P are for the two directions of polarisation. You can also see why you need to consider the grating response as well as the camera QE if you want an absolute flux calibration
https://www.optometrics.com/assets/M466Sales.png
I am not sure if the transmission grating (grism) used in the direct vision spectrograph you are using is sensitive to polarisation though. This shows typical curves for some transmission gratings but it does not show any effect of polarisation.
https://www.optometrics.com/assets/ARTx.png
The one in your spectrograph will probably be similar to the 600l/mm curve
Cheers
Robin
