Before you can decide how to set up the spectrograph you need to decide on one of two options.
You should then calculate the necessary reciprocal dispersion you need ie nm of wavelength for each mm of detector, noting that the detector is 12.5 mm in extent.
Armed with the dispersion you need, you should consult figure 6 of the spectrograph manual to find which grating and zoom lens setting will give the desired result.
You can then use this program to predict the grating angle, the exact dispersion, resolution and spectral range for your set of input parameters.
NB. Items marked with a star* are fixed parameters for the Wast Hills set-up.