Its mostly just done by eye. I work on homes so measuring short distances by eye, is an aquired skill. One thing that almost always enters mental refference, is "half of the discard tray height" the discard tray ~should~ be the height of the number of decks in play, so half od it is half of the number of decks in play, and half of that isnt hard to eye up from there, or a1/4 of it or whatever.
Two things to watch out for when estimating decks.
1. 6 deck games with 8 deck discard trays
2. The "poofage" factor the poofage factor occurs when cards are in play for a signifigant amount of time and get freyed up. After a while, particularly in 8 deck games it may only take 7 or 7.5 decks to completely fill an 8 deck discard tray, because the cards "poof" up. This effect is most devistating to TC estimation at the end of the shoe, be sure to notate penetration prior to begining a shoe, so when it apears that there are 7 decks in the discard tray, you can remind yourself that there were 2 decks cut off, and that isnt possible. This will save you from becoming a victim of the poofage factor.
I made up the term poofage factor, but it really does happen.
~FERRET~