Yep. Kinda lo tech - but hey effective if used. You could even use the smaller yellow post-it and stick it right on your computer.
Lots of players seem to want WGT to do their thinking for them.
Since I play the same ball all the time I just remember and know that right now I have 5 balls left and ball #5 has about 75% of it's durability remaining.
When I get down to 3 or 4 balls left I get another sleeve.