I usually just use
Total volume/(dilution wanted +1) => amount of developer.
Then I just fill the tank to the total volume I wanted.
Thus: For a 600ml total volume (reccomended in my tank for one 120 film) and a wanted dilution of 1+20, I take 600/21 => 28,57 ml of developer in tank and then fill with water to 600ml.
Rodinal 1+50 for 600ml total volume gives: 600/51 => 11.76ml of dev and then fill to 600ml
If I am unlucky with a strip of film and cannot print it, I consider it a failure and try and scan them instead, my experience is that you can actually salvage _a lot_ if you scan a problematic negative as a positive and then mess with the curves and levels to "normalize" them.
I like fomapan very much, one of my favorites, along with the Rollei films. (I use Tetenal Ultrafin and Rodinal mostly).
Tetenal 1+20, 7,5 mins in 20 degrees for Fomapan 100, worked fine so far for me, but I see that the dev-chart doesn't list the 400 with this dev.