Thanks,
@mshchem. That's a little less than sea water specific gravity. Now, if we had a second point, we might be able to solve for the two concentrations. I was thinking refractometer, because different salts at the same refractive index ought to have different specific gravity. In other words, any proportion of KBr to NaCl could produce 1.19 g/cc, but there's probably only one that will also produce a particular refractive index.
Know anyone with a salt water aquarium (ideally a reef tank) who uses a refractometer to check their salinity? My partner just uses a hydrometer, since the salt mixes are interchangeable for that application and it's much easier to use -- and that level of precision just isn't needed -- but some salt water aquarists are equipment fixated...
BTW, tasting like table salt is what I'd expect even with a mix biased heavy to KBr. Our "salty" taste buds are very well attuned to chloride ions; ammonium chloride ("salmiak" as used in some Dutch candies) tastes more like table salt even than potassium chloride -- KCl tastes more "metallic" (I use it, in mixture with NaCl or by itself, as a salt substitute for a while every time one of my doctors wants me to cut down on sodium). Comparatively, our detectors aren't evolved to detect bromides (or iodides, for that matter) at all, because they aren't common in our environment (less soluble, and less abundant in the Earth's crust, than chlorides).
@Mr Bill This is essentially what we're trying to do, without involving a $100,000 (or more) spectrometer to directly measure the concentrations of the four relevant ions. It might well be "close enough" to make up a mixture of NaCl and KBr, 50/50 by mole, in solution at 1.19 g/cc, and put 6 ml of that in a liter of developer, and if we don't get any better information I'll do that (I've got KBr as well as KI on hand, but KI doesn't have a known history as a restrainer, so likely isn't relevant in this situation). I've also just gotten a scale that reads 0.1 g up to 3 kg (made for brewing pour-over coffee, of all things), and still have my reloading scale that reads down to 0.1 grain (at 15.4 grains per gram, that's about 6.5 mg) though it's much more fiddly to use.
Given that a dozen or so rolls (per liter) will bring me close to equilibrium if I just don't replenish for the first six (per liter), getting anywhere close to "seasoned" condition would let me avoid messing with changing my times over/after the first dozen or so rolls. I may just put 250 ml of water in a graduate on the scale, and add equal masses of NaCl and KBr to bring up to 1.19 g/ml (that would be, what, 47 g or so of mixed salt), bottle that up and label it, and call it good. It'll be better than no starter, and likely close enough to be indistinguishable without much closer controls than I normally put on the rest of my process.
FWIW, that's approximately a 190 g/l solution, which means 6 ml is about 1.4 g of mixed salts to the liter of developer stock.