I got this picture from this thread: Wanted - Where do I get one of these? - https://forum.ih8mud.com/threads/where-do-i-get-one-of-these.1086465/#post-11870593. Maybe @ToyotaMatt still has that one he shows...
It looks like the small side the threads into the block is tapered.
normally you would measure the outside diameter of the tube. This would be the size of the compression fitting. If you have the nut and ferrule still on the end of the tube that you want to reuse, then you would need to make sure the threaded portion of the compression adapter is correct and...