I like it is generally agreed that the previous week before last counts too, and maybe to a lesser extent the week before that, so side by side with maintaining a good week minutes, you factor in previous weeks, and expect random things to happen. I purposely tried to get some players of mine under 48 mins because they played a lot the week before that, for example last week I made my PG Emil Schwartz get that 40-48 (45 to be exact) and he ended up staying proficient this week. I've done that many times; this is only something that I observed and tried. It works for me, but I'm not saying it's law.
btw my definition of perfect minutes is definitely not 66 xD