@Ohio Sean here's the URL - it's a page to download an STL file for printing a capsule I use for storing pre-measured Mytol ingredients (to make 250ml stock or 500ml 1:1), but it also has the recipe I'm using listed out on the page.
https://www.printables.com/model/863318-instant-mytol-developer-capsule
As it happens, I printed four of those capsules to give to a friend with a developing kit when he was getting into shooting 35mm. The hobby fizzled out for him and he returned it. So if you decide you like Mytol, I'd be more than happy to send you those 4 capsules to use
I keep 6 of them handy in my darkroom and refill them all when they run out. It's nice to be able to just dump one into 250ml of water and stir for a minute or two.
As for FX-55, I have recently played around with it a bit to see if I liked it better than Mytol. The classic version is liquid Part A concentrate that's basically just alkali, and stays good on the shelf indefinitely. Then you just diulte that with water and add a pinch of sodium ascorbate and a few drops of phenidone solution in propylene glycol, immediately before use.
I have also been corresponding with
@measwel for quite some time about variations on FX-55. We were both doing some experiments - he wanted to build a system where he could have a universal Part A liquid that would combine to make either FX-55 or something akin to E-72 paper developer. I just wanted to test out FX-55 to see if I liked it. There were a lot of claims by John Finch of Pictorial Planet (significant speed increases, incredibly fine grain, some kind of impalpable magic in the midtones, etc.) that had me curious. I also figured it might be worth figuring out some way to do a liquid Part B so I could easily mix it up like Pyrocat HD instead of weighing and measuring phenidone and sodium ascorbate separately.
The theory and stoichiometry has been worked out on paper to use ascorbic acid and phenidone in glycol to make a liquid Part B that is shelf stable, with a slight increase in the potassium carbonate in Part A to replace the lost sodium from not using sodium ascorbate (which is terrible for dissolving in glycol). But I haven't actually tried it yet to confirm whether the results bear out the same.
My testing is not done, and has admittedly stalled out a bit. But as a baseline, I did do some close comparisons of 35mm HP5+ in Mytol stock vs standard FX-55 diluted to 50% strength. I found for a fast cubic film like this in such a small format, I preferred Mytol. It had an edge over FX-55 on finer grain. I suspect this has to do with the increased solvent action from a higher concentration of sulfite, but I'm no chemist. My next intention was to do a shootout between the two developers with Delta 100, where grain is inherently far less of an issue. I haven't gotten around to completing that test yet.
All that said, the difference was so slight (even in 35mm HP5+) that the convenience of FX-55 might well be worth more than the marginal and subjective improvement in grain I get from Mytol. I have switched to FX-55 as my go-to in medium and large format for HP5+ where grain is a nonissue. The 50% strength version has pretty long dev times, but it's dilute enough that it seems to give me some highlight compensation even in my rotary processing setup that I use for sheet film, so it doesn't build contrast too quickly, and still gives me really excellent film speed. For FP4+ I still prefer Pyrocat. I don't really shoot Delta 100 in formats larger than 35mm.
Anyway, my
n=1 experience and $0.02 worth of rambling on the subject. Both Instant Mytol and FX-55 are great developers in my experience, and frankly so similar in makeup and results that it's a little silly to be comparing them side by side in the first place. One of those projects that has me frequently taking a step back and asking, "shouldn't I just be spending this time taking photos?"
