There is a very, very, very lengthy discussion on this topics, but it covered I think most of the issues Craft magic has - without necessarily providing final answers as you will see, since canon rules are either contradicting each other, or unclear.
Some of the points to consider:
- Is Finesse roll done with Int or Per ? (and which virtues can be stacked)
- How to compare Finesse roll result with Quality ?
- Does it makes sense to have Craft spell with Conc. duration to make thing more slowly (requiring Concentration roll), to decrease the difficulty ?
- Obviously, which are the best combination of Virtues to make a good Crafting magus ?
- How having the appropriate crafting skill can help with the end results (using familiarity bonus for example) ?
Most of these answers can be summarized at the end by "Discuss it with your group" as it is impossible to have a common agreement on this forum. There are some good suggestions also in the thread, like trading a whole season of Crafting magic to achieve high quality results.
A lot of the arguments are based on the premises that magic should not allow to surpass what a skilled craftsman can do, because it will drastically lower the importance and needs of skilled companiosn and grogs. So it is more a metagame call than a real limit of magic (trying to set a balance in a system that any way clearly favors magic).
I do not agree on this statement, since mages are already better at killing than any skilled fighter and better at healing that the best physician so the argument sounds weak to me. But I do not want to open this can of worms again.
Also, if we look at the quantity of optimal virtues you need to stack to make a truly good craft-mage, it is unlikely that there are a lot of them in the Order, so the idea of them "unbalancing" the need of craftsmen is limited. So your mage could be the first one of his kind, or at least the only one of his generation - maybe coming from a rare lineage of craftmages, having unique initiation rituals allowing the proper combination of Virtues. I believe there is good story potential to it, so should not be dismissed just for the sake of metagame balance.