This is how I do it
I count the rounds. I intuitely know where I am through the decks by the number of rounds gone, given the number of spots/boxes being played:
One spot open means 5 rounds per half-deck
Two spots open means 13 rounds per 2 decks (6.5 rounds a deck)
Three spots open means 5 rounds per deck
Four spots open means 4 rounds per deck
Five spots open means 3 rounds per deck
Six spots open means 3 rounds per deck + a bit
Seven spots open means half a deck per round, but every fifth round don't increase by the half-deck
Admittedly it gets a little tricky if the number of spots being played keeps changing but I play fairly high stakes and usually have the table to myself or perhaps one other.
AWS