Well, when I've done this with a camera, it's been either 1:1 off a 4x5 or else the other way, 8x10 down to 4x5. Mostly I was making internegs off CTs. With the 8x10s, I taped them to a sheet of white plex, held it vertically, and backlit it with a speedotron head. It's hard to take a meter reading off a ct this way, but one way to do it is to appraoch it just like slide duping. Once you nail the exposure, all images should be the same. You could use a wratten ND filter, 1.0 No. 96. This sort of approximates a gray card reading. I do this on a slide duper, and it works well. What I mostly did was to average the transmitted exposure, and test it on a polaroid. In some cases, I used the 55 P/N for the interneg. Mind you, all this is just down & dirty stuff, but it works.

I avoid internegs like the plague though, I've always hated to make them, but it's only been since we got a slide scanner that I've been able to not do them so much.

When I dupe negs, I do it by contact because the films are so slow. They are about the speed of Azo. I usually just treat it like making a print. But here again, once you get a system in place, you can group negs in batches (density & contrast) and work them through. If you used a regular film, like Plus-X or Delta 100 (seem to be 2 that are recommended), you could shoot them off a light box, or just enlarge onto them easily. It gets tricky when you try to filter out stains, or make other corrections though. Kodak recommends using Tech Pan as the interpositive, and TMX 100 as the working neg. I can't remember the aim points right now, but you can use a densitometer to fine tune all this, and a fairly accurate match of the original negative can be had by someone who knew what they were doing.

Back when labs used long roll contact printers (some still do), I think enlarged negs were more commonplace. We still use a lab that dupes up to 8x10 off our 4x5s for murals. I think they use a stat camera with an illuminated baseboard for this though. I imagine that would be the best for this sort of thing, makes me wish we hadn't of surplused ours now....