I'm new to reloading, so I'm no expert.
But what I am an expert on is not being able to see the case in the Dillon case gauge.
So what I did is take a set of my automotive feeler gauges apart and used the little ruler that they all seem to have,,,or you could use a fat feeler gauge that's not used,,, and used this as a dedicated straight edge to "FEEL" the case.