If you are really looking to get extremely accurate in estimation for other purposes other than counting, its actually not a bad idea to practice with various types of cards. Cards will vary in thickness even within the same brand due to many different factors. The best trick to learn is on the fly contrast and compare. If you can find a control size that compares to the cards you are playing, you can estimate any kind of card. The highest degree of estimation comes from more than just trying to memorize deck and fraction of deck sizes. It also involves calibration. Of course if you are just looking for decent estimating skills for just counting, there is no need to go that far in training.