I have a TD-504 unit with manual. I am not sure what the differences are from the TD-500.
To use it you turn it on fro a few minutes first so the light source can stabilize,. Then I zero the reading on the film base between frames, then read off density above film base.
I checked the gain of mine with a step wedge. I found it made sense, so I have never needed to fiddle with the gain or slope potentiometers on the back.
Mine has a fillter barrel for c-41 readings. I use the yellow setting for reading B&W films.