Best Practices for Scanning Film Negatives with a Digital Camera (DSLR or Mirrorless)

Spot meter on space between frames should be sufficient if the spot does not cover too much of the two frames. It’s imaginable that the reading on space between two dense/dark frames can yield another result than between two underexposed/light frames, so I recommend careful testing. So why not, give it a try with two underexposed frames. Or create a blank frame on next roll (1 picture underway with lens cap on).

You can get a pretty good idea by just making use of the highlight warning on Live View if you have it. If you get just a hint of it on the base layer between or around the frame, or any unexposed part of the film that should be OK as the RAW file will have more range than the range of the jpeg that you will be seeing, or just knock down the exposure by half a stop from there. That saves you comparing different spot readings. Presumably you’ll be using manual exposure for this and from that point on.

Once you’ve arrived at your perfect exposure by however means (and you can always refine it from experience of course) I’d also suggest taking the film out and taking another about 3 stops down, same aperture, just to see that you have evenness of illumination. You can also use the highlight warning to give you a visual indication of that in fact, just increase the shutter speed from the one you’ve settled on (with film in place) until the highlight warning starts to disappear. It’s almost bound to start to disappear from the outside of the frame first, but hopefully over a very small range of exposure.