These can also be used in the Level 36 +2 <skill> rings tht cap at 55. I just made 2 of them and then used them to make an Ametrine and a Deft ring. No clue if they help +1 chances for the rings, I would doubt it though.
It works like this, i will use the Ametrine ring as an example as it is the one the crafters like me will almost certainly craft the most in the game.
Ametrine Ring: Ametrine x1 Mithril Ring x1
Deft Ring: Ametrine x1 Mithril Ring+1 x1
[HQ]Deft Ring +1: Ametrine x1 Mithril Ring+1 x1
So to summarise, a Deft ring is an HQ of the Ametrine ring, and the Deft +1 ring is the HQ of the Deft Ring and in order to make a deft ring you will need to use a Mithril Ring +1.