Hasta La Motion!

I’ve seen some mixed information on the use of filmback measurements so hopefully someone here could help clear this up !

If you have shots which are done on a digital format camera of some sort, how do you find out what the equivalent filmback length is, and do you actually need it for a good solution ?
(other measurements have been taken such as focal length etc)

And secondly, what happens when you have shots that have come from a 35mm adaptor fitted to a digital camera, I presume the filmback is then coming from the adaptor ?

I’m trying to find out because I have some shots which are closely matched but I’m presuming this will create an even more solid match for me ?