Also, different manufacturers of decks can have different thicknesses so if you truly want to attain 1/4 deck resolution you'd have to take that into account (cards also expand the more they are handled and shuffled).

I have Aristocrat and Paulson brand cancelled casino cards from the same casino I played at recently and you start to see about an 1/8 deck difference when 2 decks of each brand are stacked side by side (Paulson are thinner). And Bee are similar to Aristocrat. So that will eat into your 1/4 deck accuracy when you get into 2-5 deck estimation.