You can do a main movie backup of several of the Harry Potter movies (keeping the primary audio track only), and the result will fit on a BD25 with room left over. Keeping in mind the length of those movies, and their "compressibility" (action, dark scenes), there's not much bitrate to spare.
A crf 18 constant quality encode of any of the above will not save much space. Surprisingly though, you'll see significant variation (a few GB or so) in "compressibility" even among that limited group. So again, it's essentially unpredictable.
Which is why I think it's wiser to use constant quality encoding rather than setting a target size. Find an acceptable quality level and let the file size be whatever. The exception is encoding to BD25, which is ample space for a main movie backup of an average length movie, IMO. Encoding times don't concern me, as I have a 6-core machine for this and can queue up several encoding jobs at a time to do overnight.
I can certainly see why some people prefer to do a straight rip with no re-encode. I have my original Blu-Rays stored away. If and when I get a 4k 80 plus inch display, I'll look into this again and if necessary go with straight MKV rips on a NAS. As it is, I have most of my Blu-Rays re-encoded in MKV container on several 2 TB external hard drives and can play movies directly with my TV's onboard player.