Wow, I never thought I'd see you defending Soviet gear,
@Ko.Fe.; you're normally the first to belittle them!
Honestly, at this point, I've lost count of the amount of Soviet lenses of all flavours that have passed through my hands over the years. Nothing like as many as Brian, of course, but... a lot. And all of the FED and Zorki ones have consistently back-focused by the same amount on an LTM camera, whether it be Leica, Canon, Nicca, Leotax, etc.; the problem is known, understood, and measureable.
Very old test from ten years ago:
Soviet Lens Tests, Redux. by
Tony Gale, on Flickr
Three different lens designs (J8, I26m, I22), three different sources, three different factories (KMZ, KOMZ, FED). All running to exactly the same spec and back-focusing by exactly the same amount compared to a Leica's rangefinder (shown as accurate using a Summicron in the same test). And this is at f/4; the difference is
much more pronounced at f/2 on the Jupiter 8. Same thing happens the other way around: keep a FED calibrated to the Soviet spec and put a Leica or Canon lens on it, and it'll front-focus by the same amount.
You do raise a valid point about the lack of haze and separation in Soviet lenses - I was dwelling on that the other day while cleaning the haze out of a 90mm Elmar. So many Leitz lenses from the same era are hazy as hell, but Soviet ones are not. Instead, they have their own problem: lubricant that turns to beeswax and seizes the whole lens. Stripping out that old lubricant is the first "fix" I taught myself to do with lenses and cameras, and I ended up doing it for a lot of friends over the years as a result. It's an easy fix, sure, but it's still something you have to sort out alongside the RF calibration issue. This is another reason I'd suggest OP sticks with his Contax J8M and picks up an adapter instead.
As for affordable LTM 50s: I'd go for Canon's collapsible 50/1.9 Serenar, personally. Yes, it's more expensive than an "LTM" Jupiter 8, but not by much in the grand scheme of things. And looking through eBay here in the UK, the 50/2.8 Canon is only £60, which is basically the same price as a "good" Jupiter 8 which is unlikely to focus correctly anyway. Some of the Leonons also crop up for relatively cheap if you're patient.