The classic palm is more of a journey than a destination. You might be able to palm a coin confidently in three to six months, you may get it in three to six weeks. However, you will still be learning things about it three to six years from now.
You are correct in thinking that the sweet spot is different for each person, and if you are following the advice of your article there's a good chance that you are close. If, though, you are suffering from 'claw hand' it means you are gripping the coin too hard. This can be for several reasons; a lack of confidence, a misalligned sweet spot or the wrong size of coin. Try with a slightly larger coin and see how you get on. It's far easier to learn to palm with different sized coins once you have learned the hold with the ideal size.
The best advice has been given, so I will merely summarise; palm a coin all the time. Practice switching from whatever fingertip rest you use into palm and back. Learn to forget the coin.
Good luck - coin magic with normal coins is a very useful skill to have when you feel the need to perform on the spur of the moment
Take care, Ian