Wow! That's a bit more detailed for my pee brain to comprehend all at once, but thanks. I'm looking at one or a set of Conkin p-size graduated ND filters and just hand holding them, at least for now.

Basically, the widest field of view isn't when a camera is in landscape orientation, it's across the diagonal. You can measure that, and then half it, to work out the centre of the optical axis. We can measure or work out the distance from the pin hole to the back, and then using some trigonometry work out the angle of light from the pinhole to the back corner. Then project that forward and see if it will clear your filter holder.

Or just try it, and see what happens, which is what I first did.

You can use the graph paper on the diagram (it's to scale each small square is 1mm, so 1cm boxes) to visualise your filters. Cokin P is 84mm wide, so as long as it's no more than 30mm away, it should easily work, allow for the filter holder side being the way. The red line (top left on my diagram, stops 40mm away from centre line.

Okay, I'll have to take some time to absorb all that. Basically, I look at where they middle of the camera is - and then consider the upper half for one exposure and the lower have for the other exposure, each time covering up the part that's not being exposed at that time. I am almost always shooting in portrait mode. I did this with some success with a still life set up, but I didn't quite get the placement correct. But the idea generally works, I just need to practice it.

I am mainly concerned with the exposure and how not to either under or over expose when I'm taking two images on one piece of photo paper. You're ND filter info helps with that.