Flawless Magic and Mastered Spells

The Flawless Magic virtues includes "Further, all experience points you put into Spell Mastery Abilities are doubled."

And the Mastered Spells virtue says "You have 50 experience points to spend on mastering spells that you know."

Would that mean that when creating a character with Flawless Magic, you can select Mastered Spells once and effectively get 100 (rather than 50) experience points towards further spell mastery levels?

I would say yes.
It seems fair enough (as fair at it can get with Flawless Magic involved - because that virtue is very good!), plus easy to handle.

I use Meta Creator for all charcater creation, but as I haven't tried this cmbo I'm not sure how MC handles it. But whether you say you get double exp for mastery or that mastery costs half to increase should yield the same result.

I often take Flawless Magic, it really is my favourite Hermetic Virtue. I love Mastery and I preper spellcasters to device-makers or ability-users. It is best for using lots of spells, magi concepts using only few spells may not get the same benefit.
I've never taken Mastered Spells though, I prefer more actively usable virtues rather than extra exp virtues. I feel the same way about exp-boosters like Book Learner or Free Study. Although I like Independent Study a lot - the bonus Story Exp is a nice bonus for extrovert magi. Plus the bonus exp for Practice - whicn done for spell mastery with the Flawless Magic magus...kaching!

Ok, I tried with MetaCreator to whip up a Flawless Magic + Mastered Spells magus.
When I use "change mastery" function to raise mastery levels, the exp counter starts from score 1 cost 0 (due to FM virtue) and tracks the cost normally, minus the 5 free points from FM.
But the tracker for how many points spent in Mastery as a whole, for the MS virtue only counts those spent exp as half. So a Mastery 4 spell listed for 45 exp and a Mastery 5 spells listed for 70 exp sums up to 58 Mastery exp, effectively halving the cost.
There you have it

Yes, the xp you get from mastered spells is doubled by Flawless Magic following the rules as written.

I think this combo follows the same principles as if you had Linguist and Well-Traveled. The extra exp you get you cna spend in Languages for extra bonus. Or Skilled Parens/Gild Training and Affinity [Art].

Real problems arise from trying to simulate the bonus from Elementalist or Secondary Insight, since these need you to know the exact proces of exp gain, how many times and how many at a time. You can't do this without developing the character season by season. Which is why you don't get any bonus from these during character creation but only during play. Same goes for Book Learner, Adept Student, Free Study, Independent Study.

For a long time there has been a canon optional rule for Elementalist that provides the experience, providing the simulation so you don't have to figure one out. It's somewhere in HMRE.