Estimation is a secret weapon that lets magicians create miracles. It is not as difficult as you think. I have been using estimation for over 30 years. When it works, it is nothing short of a phenomenon.
My nw eBook lists the requirements, explain the basics of estimation, the difference between in the hands and on the table, practice tips and finally effects you can do with estimation. Some of these techniques and effects are fairly easy to do, others are not.
Requirements:
Good eyesight: is a must. If needed, you can where glasses, contacts etc.
Good lighting: If you can’t see the cards clearly, your results will not be good. This means the edge you are looking at has good lighting on it with no shadows.
Deck in good condition: if the cards are bent, crimped, buckled, etc. it is almost impossible to get accurate results. Use a deck in good condition or don’t do estimation. Watch when the spectator shuffles your deck. Some spectators are very rough on cards.
Same deck: Cards can be different thickness from 300 MGS to 330MGS and this makes a big difference. It does not matter which type of deck you use, but once you choose a deck stick with it. I would not use some off brand. There is a greater chance they might go out of business. Bee, Bicycle, or if you have your own cards made are all good choices. The odds of a spectator having a deck with him these days are very low. Use your own deck.
Squared up: The deck must be squared up. The neater the better.
Practice: Lots of it. This is a visual skill and some of a kinesthetic feeling. Some days you will be dead on, other days you will be off. Normally when you are off, it will be off by the same amount.
Example: two cards less than the number of cards called for. If you are really having a bad day, take a break. I believe in the effect of negative practice. Start again later. This normally helps. To get really good at estimation you need to practice it daily. There are many more tips in the Practice section.
My new eBook goes into much more detail.
It is available now at: https://www.lybrary.com/estimation-a-visual-deep-dive-p-926691.html It is only $14.97 until Oct 9th, then it will be $19.97
It is 43 pages with lots of photos to help explain techniques.