Do what I did. Go to Walmart and buy 21 decks of cards. Go home and stack them on your coffee table or someplace where you can stare at them all the time. Stack them in piles of 1 deck, 2 decks, 3 decks, 4 decks, 5 decks. Then take a pile of 6 decks and practice cutting out 1 deck, 1.5 decks, 2.5 decks, etc until you get really good at estimating decks played. I also purchased a 6 deck discard tray like they have at the casino and blindly grab random handfuls and stack them in the discard tray. Once in the tray guess how many cards are there. If you leave the cards in a specific order it's easy to tell how close you are. You'll be amazed at how accurate you can become in a relatively short amount of time.
I'm not a big fan of computer simulations for deck estimation. Good luck.