I'd suggest the rules are already there...just make a declaration. "I'm using two charges!" Tag it for your +2 and mark two off. Using more than two should probably remain dependent on how the GM / table likes to use / stack declarations and aspects.
UmbraLux's suggested case, above, using a second use of the item as the
cost for making the declaration (rather than a roll or FP which is usually necessary) is perfectly costed, imho, and a simple and elegant way of handling this. If you wanted to use
the rest of the rings as well, I'd probably allow the player to make two declarations: "I Use Two!" and "I Use The Rest!" and cost those declarations accordingly for one or two +2 tags, respectively.
I like the idea of simply using declarations to model this, then compelling those aspects if the player tries to use the item again.
You wouldn't even need to use compels if you consider the cost of making those declarations to be crossing off extra uses. Then there's already rules for additional item uses.