If you are going to do a $20 chip shouldn't you also consider a $10 chip as well? I know in my game ( a $10-20 limit game) we love a $10 chip. We use a $5 and a $10 chip. I also have some $25 chips but they hardly ever get used. If we had a $20 chip I think it would get used as much as the $5 and $10. Just a thought.
I would consider a $10 if people were interested in getting some.
That said, the conventional wisdom holds that if we do a $20, we should definitely not do a $10. Generally, you should have a factor of four or five between your denominations ($1, $5, $25, $100) Your game is, of course, an exception, and that's OK, too.
When they were introduced in 2004, the Key West denominations were $1, $5, $25, $100, $500, $1,000 and $5,000. (Those are very traditional old casino denominations.)
I've tried to be careful about adding denominations that are traditional (the $2.50 for blackjack, for example). I also think about which denominations will appear on the table together, and which ones won't. (nickels for micro-cash and $25,000 for high-value tournaments).
The $20 is popular at cash tables, particularly the cash table set up for those who bust out of a tournament. A $20 chip at the cash table ensures that the $25s used in the tournament can't be used at the cash table. So I got several requests for the $20s, and made some in 2013. So we have the $20 inlay, on file at Classic Poker Chips, ready to go. All we have to do is tell them what color chips to put it on.
And you see how difficult it will be to decide.