The way we've dealt with this is based on the way in which Persephone's Secondary Insight was handled in Magi of Hermes. First, assign all xp as normal. Second, work out bonus xp in the Elemental Arts by working out how many seasons have been spent on each Art based on the assumption that the magus was studying from a Source Quality of 8 (rounding up to the nearest whole season).
So, let's say that you initially assign 6xp to Aquam, 28xp to Auram, 36xp to Ignem, and 10xp to Terram for scores of Aq 3, Au 7, Ig 8, Te 4 (the rest of his Arts don't matter at this stage).
6xp in Aquam implies 1 season of study. This means +1xp Au, +1xp Ig, +1xp Te
28xp in Auram implies 4 seasons of study. +4xp Aq, +4xp Ig, +4 xp Te
36xp in Ignem implies 5 seasons of study. +5xp Au, +5xp Aq, +5xp Te
10xp in Terram implies 2 seasons of study. +2xp Aq, +2xp Au, +2xp Ig
So, in all the character has +11xp in Aquam for a score of 5(2), +8xp in Auram for a score of 8(0), +7xp in Ignem for a score of 8(7), and +10xp in Terram for a score of 5(5). As it should be, the Arts that were studied less have had a greater boost than those that were studied more --- Elemental Magic is great for levelling out the elemental Arts.
The character has had 36 bonus xp in total; a little less than he would have got if he had spent 4 Virtue points on an Affinity in each of the elemental Arts; and a fair bit less than if he'd taken Skilled Parens (for 1 Virtue point). However, Elemental Magic is not really comparable to Skilled Parens since, like an Affinity, it is a Virtue that keeps on giving.
Note that it is entirely possible to get more xp during character creation with this method by tweaking the xp allocation to the elemental Arts. I've deliberately not done this in the example; but what I probably would do is reserve a small pool of xp (5-10) during the initial allocation of xp to Arts and Abilities, and use it to 'top off' those elemental Arts that are close to the score boundaries. For example, another 2 xp in Ignem would push it to 9 without adding any more xp to the other Arts (since ceiling(38/8) is still 5), and another xp in Terram would similarly boost it to 6.
Mark