Hello Kate,
I use this slit as a matter of course, primarily as a means of getting the best definition of the continuum that I can with my equipment & set-up. Although it means 4 measurements instead of 2 it’s a very useful piece of kit just for this function alone. I don’t use it for photometry though I have processed some of my measurements following the examples given in the Christian Buil references Robin has given, just to see what sort of results were produced – the instructions in the references are straightforward & clear.
john s