Just don't forget to check out your camera and filmholder system very well for light leaks before putting it to the test with real film Few things in photograhy are more troubling than a 12X20 negative that has been ruined by light leaks!!!! Believe me on this because I am working on a second doctorat on the subject!!!
How to check? Put a flashlight in the camera and in complete darkness shine it around all over the bellows, then put a holder in the camera and do the same all around the holder, then remove the dark slide and shine the light directly through the baffle. The assumption should be that something is wrong and that there is a light leak rather than the contrary.