Finn

The practical way out of your actual situation, when you are halfway through a roll and the light drops: leave the dial alone and change something else.

Open the aperture, drop the shutter speed as far as you can hand-hold, brace against a wall. On a 400 roll indoors you are often only a stop or two short, and those are available without touching the meter setting.

If it is genuinely much darker than that, the other honest answer is to finish the roll quickly on anything, rewind it, and load a faster one. A few wasted frames is cheaper than a dozen unusable ones, and much cheaper than a roll you then have to guess the development for.

You are not missing a system, because there is not one. There are several, and they overlap badly.

Machine ports cluster around a handful of sizes that come from different traditions — imperial in inches, metric in millimetres, plus each vacuum manufacturer's own hose fittings which are designed to lock to that maker's accessories and nothing else. A 2.5 inch port and a 58mm hose are close enough to look compatible and far enough apart to leak forever.

The practical consequence is that chasing an exact adaptor is usually the wrong strategy. What works is picking one hose size for the whole workshop and adapting every machine to it, rather than adapting the hose to each machine. One size, several machine-side adaptors, all of them staying on their machine permanently.

That turns your problem from "find the fitting" into "make one fitting per machine, once".