This cannot be used in combat.
"You attempt to use the Perfect Life Stone."
"You must end your current action before using an item."
"You can't discharge that item."
So i sheathed my sword and waiting 1-3 seconds.
Then i used the stone. Although the delve was 254, the heal effect was only for 190 hit points. Hunter, 50, rr5.
There appears to be a 3 minute reuse timer before you can used the diminished Stone or another Stone.
The Weakened Life Stone has a delve of 154, but only healed me for 100 points.