Utter brainfart, for a moment, I confused this with secondary insight
Although ErikT is, IMO, "more right", I tend towards @Daelnoron 's position.
For 2 reasons:
As noted, it's easier with excel. That's a shitty reason, I'll admit, but here it is
It allows you to make elementalists that are more focused on a given element, which I believe to be a staple of the genre. Kinda like Aaang, despite being the Avatar, usually fell back on Air. This, in turn, allows for more diverse elementalists, which, IMO, is good.
I feel like a general push against stacking xp bonuses might be a sensible policy, even if it's unlikely to be particular problematic in this instance. That said, I agree with the comments about allowing affinities to apply helps support elementalists specialised in a single element.
It is not that clear to me An affinity in Ignem might simply mean that a magus understands the "language" of ignem more readily. If a magus views all elements as connected, it is not unreasonable more insights in one Form (perhaps due to an affinity) translate into more insights in another.
So, I think that the cleanest solution is just to say that yes, if you have an Affinity with Ignem, you learn the other Elements better when studying Ignem.
I would add a tangential comment: it may be a good thing to clarify what it means that an Affinity increases a character's Advancement [pre-errata: Study] Totals in that Art/Ability by one half. In the sense that it is not clear what an Advancement Total in something is. ArM5 defines an Advancement Total as Source Quality +- xps from V&Fs, and a Source is not necessarily in one Ability/Art (e.g. Adventure or Exposure).
If an Adventure has Source Quality 4, and a magus' whose sole Virtue is an Affinity in Ignem puts "2xp into Creo and the rest into Ignem", how many xps in Ignem does he get? And if he puts all his xps into Ignem? I know how my group plays it (exposure and adventure Sources are effectively "split" into multiple Sources, one per "xp destination" - i.e. Ability/Art/Formula etc.) and I know other groups play it in the same way, but it is far from obvious.
Clarifying this first might make it easier to word Elementalist so that it produces the desired effect, whatever that might be.
In a silly way, Elemental Magic is Affinity. When you study Terram, you have Affinity in Aquam, Auram, and Ignem. No matter if you apply Affinity to Terram before, or apply Affinity to Aquam after, you are applying 50% on 50%. Where is the symmetry broken?
We have to take a step back and look at the overall effect. If you have Affinity to Terram, how much time do you spend studying other elements? Would it change your study plan if Elemental apply before/after Affinity?
Where the symmetry breaks is for summae. The 50% of 50% either applies before you switch out of Terram summae, making Aq/Au/Ig closer to their summae cap. Or it applies after, allowing you to study those summae for a longer time and boost Terram further.
If the image of Elemental Magic is pushing Arts to score 30+, Aq/Au/Ig cannot gain from an Affinity with Terram as we'd rather have Terram grow faster when studying other Arts, after Terram summae is capped.
If the image of Elemental Magic is reaching score 20 faster for all Aq/Au/Ig/Te, an Affinity with Terram should boost Aq/Au/Ig.
My vote is to apply Affinity to Terram when studying Aq/Au/Ig, as score 30+ are more mythic. It also makes Initiation into EM a worthy goal for mid-level magi.
The only term that IMO should be used in this virtue is Advancement Total. Halve the final Advancement total for the main art and apply it to the secondary arts. It's clean, it's simple, and it incentivizes focusing on your main Form (presumably the one you have Affinity with).
The limits of Summae should probably apply to all of the Forms involved, however. Studying a low level Auram summae shouldn't do much for your high-tier Terram score.
You have been trained in the ability to manipulate raw elemental Forms (Aquam, Auram, Ignem, and Terram), and view them as a connected whole rather than four separate Arts. Whenever you gain experience from a source dedicated to one of these Arts, you gain half the Source Quality in experience points in each of the other three Arts. Virtues and other modifiers that affect the Source Quality (like Adept Student and Book Learner) do affect the number of experience points gained in the other Arts, while Virtues and other modifiers that affect the Advancement Total for the Art you are studying (like Affinity) do not. For example, if you studied Ignem from a book with a Source Quality of 13, you would gain 7 experience points in each of Aquam, Auram, and Terram. If you had Book Learner, the Source Quality would become 16, so you would gain 8 experience points in each of the other three Arts. On the other hand, if you had Affinity with Ignem (but not Book Learner) you would gain 20 experience points in Ignem, and 7 in each of the other three Arts. Affinities in the secondary Arts do affect the experience points gained in those Arts. For example, if you had an Affinity in Aquam, you would get 13 experience points in Ignem, 11 in Aquam, and 7 in the other two Arts
The proposed wording does not clarify how the virtue applies during character creation, since it uses Source Quality (which is not used during character creation).
An affinity in Ignem might simply mean that a magus understands the "language" of ignem more readily. If a magus views all elements as connected, it is not unreasonable more insights in one Form (perhaps due to an affinity) translate into more insights in another.