However, in those cases, you don't move/shift the body to have the taking lens be directly where the viewing lens once, was right? This will affect things if a. subject is up-close, and b. angle is not straight on, or c. subject is irregular in form.

|_\ .... <-- c330
|=|: ------------------------------ *
|=|: ----------------------------- ** <--- xmas tree / pile of cannon balls
.-- _____________________ ****______________________________

I'm throwing out this example above ^^.

However, seeing as you're already there with a loupe and are familiar with the focus of the film plane, this might be entirely too elementary and already discarded by you as a possibility. :-)