How to use a knife to cut through stone.

I still use the Craft Ability to limit it, not the Finesse Ability. The Finesse Ability I use for the craft magic roll. I use my interpretation of the flavor of this rule:

I figure just having a good Finesse score doesn't mean you have any idea about how to perfectly balance a sword, bend its steel just so, etc. If you understood, you could do it with your craft magic, though. So develop your Craft Ability if you want a higher cap. This doesn't stop craft magic from being good in the hands of someone who wants to use it. I've actually asked for this limit to be placed on my own craft magic magi because I know just how abusive things get otherwise just by spending a little time being reasonable. It doesn't take that many rolls to get 1,1,... Sure, it takes hundreds. But you can easily get the materials, and you could cast a spell once a day for a year and expect to end with a roll plus skill in the ballpark of 50. Meanwhile you could probably sell some of your scraps, many of which are bound to be fantastic.

Looking up Covenants p.52 Stone-Cutting Knife, you also see, that the device, as about all other enchanted devices of ArM5, does not come with precise instructions for use.
So the details of the device's use - how to trigger a D: Mom use of it, how to find out that it needs to be triggered anew, how to make sure it is not accidentally triggered, how to use it for a specific cut, and dozens of other aspects - are left to the troupe.

In such a case an author better requires the reasonably well defined Finesse Ability for Craft magic (already in Covenants p. 49 box, and afterwards HoH:S p.60ff) to operate it, than a Craft Ability completely defined and delimited by the troupe.
As this may fit better or worse with a troupe's handling of Craft Abilities, we have already

So troupes are invited from the beginning to modify the Ease Factor for different tasks, effects and devices. And some such modifiers are later codified (see HoH:S p.62 box Finesse Bonuses and Penalties for Familiarity).
So, if you put a lot of work into the Stone-Cutting Knife of your saga, you are nearly supposed to adapt the rules for its use as well.

Cheers