The best places are in
Steve Draun's book. (Link below for index of book)
http://archive.denisbehr.de/archive/resultbook.php?book=14&highlight=4334&edit=#4334
This is probably the exact thing I do. Plus the book is straight up amazing.
You should also dig around in Expert Card Technique. There is a push off taught in there that is also very similar.
The problem is that when I started learning the push-off, I didn't know people had a clean push off. Most of the ones I saw was to push off two cards and clean up the edges as your right hand comes to grab the card. I thought that sucked. So I spent a year perfecting the one I do. There is no secret technique, just a knack that takes a long time to get from 90% to 99% down.
the only thing I can tell you is that the main part of the push off isn't your thumb pushing the card. its your middle finger on the deck that regulates how many cards get pushed off.
Good luck.
-Tony C.