Thanks everyone for all of the replies. The 2 brands that i have been practicing with are slightly different thickness. When the two 6 decks are side by side there is about a quarter deck difference. Re- marriedputter, after the first couple of times the full 6 decks were stacked in the discard tray my eyes did adjust for the difference between them so i'm hoping it will be just as easy in a casino setting.
Re- Stealth, one method i have been using for deck estimation is what you mentioned by numbering the full 6decks from 1-312 I was cutting 1, 2, 2.5 decks etc off the top then flipping the card over to check what number it was to check my accuracy. I was recently given a better idea which more simulates actual play by cutting off random sized chunks off the top then estimating how many cards are left standing. That way i'm not always looking at the full 6 decks first.
Re- Tthree, sinking the discard tray into the table is quite sneaky as is using different trays. Just goes to show what lengths they will go to.