I am a little confused.
Flawless Magic doubles every XP you put into a Spell Mastery.
Charm Magic (Mystery Cults - Merinita Folk Mystery) says you can use mastery XP to add extra charms to your already known spells for one XP per magnitude of that spell.
I am not sure that if one has the Flawless Magic virtue wether the XP put into associating charms with spells doubles or don't?
all experience points you put into Spell Mastery Abilities are doubled.
Instead of increasing her spell mastery abilities, a maga may devote mastery experience to associate additional charms with her formulaic spells.
It doesn't look like Flawless Magic would apply. It double experience put into those abilities, but the experience is put into charms instead, so the experience is never placed where it gets doubled. Had Flawless Magic said something like doubling all Source Qualities when mastering spells, that would result in a different interaction.
Not sure if it changes anything, but the DE has changed Flawless Magic to instead double the Advancement Total for Spell Mastery abilities.
(Which is amusingly enough back to how the virtue used to work in the first printing of the 5e rule book, pre-errata.)
The way I read it - "Instead of increasing her spell mastery abilities, a maga may devote mastery
experience to associate additional charms with her formulaic spells. For each charm, this costs her 1 experience point per magnitude of the spell being modified.", we are indeed talking about mastery experience for a specific spell. Since charms are associated with formulaic spell when the spell is invented, spell mastery experience is then used up to add additional charms after the fact. So if you were taught a 3rd level formulaic spell by your parens during apprenticeship, you could gain spell mastery xp after initiation into charm magic to enable charms for that spell, in lieu of spell mastery abilities. As an example, you started with 5 xp, since you have flawless. You took Penetration. Then you add 3 xp towards it from an adventure sometime after the initiation, which gives you 10 for a total of 15. Instead of gaining a second mastery ability, you could decide to add 2 charms to the spell for a cost of 6 xp. You would later need a total of 15 + (2 charm * 3 magnitude of the spell) = 21 xp to then get unlocked multi-casting, if you were inclined to that.
I hadn't double-checked there. I would say that definitely changes things. There is no such thing as "mastery experience" until it has come from somewhere. For example, you might do Practice in Spell Mastery. This gives an Advancement Total of its Source Quality (5) x2 = 10 (assuming no other modifiers) experience toward Spell Mastery. Only at this point are there experience points to be applied. So with the DE update, there is no way for Charm Magic to function here without Flawless Magic applying.
Of course, now Flawless Magic will not apply to things like the Learn from Mistakes mastery option at all. Experience gained without an Advancement Total wouldn't be multiplied for either Charm Magic or Spell Mastery in general.
In any case, the xp spent on Charm Magic shouldn't affect the xp needed for Spell Mastery, same as the rules for experimental philosophy in regards to Medicine/Philosophiae/Artes Liberales.