I mixed Mytol several times thru the years, finding that stored in a closed full glass bottle to be no problem, but once opened live seems to be pretty short. I use a film leader in a 135 film container and add Mytol 1:1 and check for dmax after 10 mins. If the leader looks too thin, I rather mix a new batch. Last time I used it I collected enough films to use up 1 l as 1:1 mixture, which is 8 films in a Jobo 1520 one-shot.
I really like it with Delta 400 and 3200 (@1600)