Hi,
I will assume that the character does not have access to hedge traditions. (If he does, the best craft magus is a Learned Magician: Learn Charms or create Chartae that provide minor powers that duplicate Hermetic craft magic spells. Then concentrate on Finesse, and learn more Charms that make it impossible for you to botch your Finesse rolls, and that boost Finesse. All of the final effects are mundane, so the SuMa charms can lapse without consequence.)
So, a Hermetic craft magus. You obviously want Finesse. Lots of it. Spell Mastery is an expensive virtue for you and since you will almost always craft under non-stressed conditions, it does not provide one of its most important benefits. So, we have Cautious with Finesse, because that roll is always stressed, and, of course Puissant and Affinity. That's the basic stuff.
Now for the add-ons:
You want to start with a maxed Finesse, so virtues that grant extra xps during character creation are always good. You might want to start a little older, to boost the max you can start with RAW (in sagas that use that rule), especially if you don't expect to find books about Finesse. Strong Faerie Blood helps with this. But the usual suspects here are all good: Parens, Gild, etc.
SFB(dwarf) does not help you, since although you are crafting, the total does not technically include a Craft skill. If your GM rules otherwise, however....
Faerie Sympathy: If your troupe allows this stuff, and they probably shouldn't, a high Faerie Sympathy of the right kind can boost some, maybe even all of your Finesse rolls. Strong Faerie Blood helps with this, of course. Plenty of gods can work (even if they are of some other realm, there's always a faerie version too....)
Essential Nature: Does Finesse in your saga sometimes involve Per and sometimes Int? Take the Minor or Major version of this virtue, and never worry about it again. This can provide a +6.
Craft Magic used for alchemy can do things like restore fatigue nearly instantly (poorly worded guideline suggests that the Mighty version (which your character won't be able to perform anyway) might be able to bring a character from Unconscious to Unfatigued, which I'm pretty sure is not the intent). No additional virtue is needed for this, but you probably want enough xp in Academic Abilities to justify this, and more xps in the formulas you know. Creo Magic comes into its own here, to replace an apothecary or similar; a few spells should be able to create a few lifetimes' worth of material. A craft magus definitely wants to be able to do this stuff.
Confidence cannot hurt, providing +3/pt to a critical roll, and there are various virtues (and flaws!) that can provide more of this.
Even without Faerie Correspondence, you can reasonably start play with a Finesse Crafting Total of:
+6 Essential
+9 Ability
+2 Puissant
+1 Specialty
= 18+die.
Not too shabby.
(SFB for age 36-40, Essential, Pu/Aff/Cau Finesse, leaving 2 (or 5) more vps.)
Anyway,
Ken