In the meantime I've switched over to Mytol, so I can mix 0,6 or 1 l, or what ever I need, I usually use it 1:1, so that is 150 ml stock per film. Earlier I used the 5 l packs and never had problems, stored Xtol in brown glass bottles (2.5 l + 2* 1l, when both 1 l bottles were empty the 2.5 was split up to refill both bottles).
Using some scrap film for testing d-max is not a bad way, I use an empty clear film container as developing tank.
Xtol/Mytol is my favourite for Delta 400+3200 films.
If using dilutions like 1:2, 1:3 it is best still to use 150 ml stock for 36 exp film or 120.
Wolfram