64 can't easily be done in an sensible way as I use an equation to get the amount. This because I wanted to make sure it scaled at least somewhat properly as the rp progresses. Now, try to make some math that is easy for the gm to follow, to get 4 from 5 without it getting ridiculous if you add more points into that stat. (thus not magic skill-1=focus counters).
I may slow the decay down, by having it decay once every 2 rounds. This would mean that she would have lost only 5 focus counters in that fight as opposed to 10. This means that instead of having to use gouged eyes 3.33 times in that fight (counters lost/counters gained= 10/3=3.333) she would only need to have refreshed it 1.6666 times (again points lost/points gained=5/3=1.666) and the longer the fight goes on the more impact this has if I am not mistaken.
I can also make it that the amount only decreases if the amount of focus counters didn't increase. This effectively makes me put 4 counters on my target (putting 4 counters and then removing one is the same as putting 3 counters and
not removing one). I think I would go with this one if there was any sign of a character with an Proliferate like effect. However as that is not the case I am not sure if doing so is the right call because I then basically force all future abilities that will create focus counters to have the amount I say +1 which makes it very hard to make exactly 1 focus counter. As I either need to specify that this turn no focus counters are lost or need to add 1 and specify that a focus counter is still lost this turn which just sounds really backwards.
For those that don't play MTG or have forgotten what it does, here is a card that explains it:
http://magiccards.info/som/en/144.html
Oh, if you think that having the amount of focus counter scale up on magic skill without dividing it is sensible think about it this way. By having the counters scale up on magic skill I "merged" 2 stats. This means that each point I add to magic skill is basically doubling in value and thus I can more easily afford doing so. Which you can already see in her current stat distribution as almost all relevant stats are above avarge. In fact the only stat that is not above avarage that is sort of importand is agility which should not be a problem as there are only 3 characters with as much magic skill as Lux has magic defence and only 1 character has more magic skill then Lux has magic defense and this character is mostly a support with even less agility then Lux (yes, a blind person is better at evading attacks then this character). And when it comes to armor it is even more in her favor as not a single character has more or equal amounts of strength as her armor. The closest they get is 5 which is 1 off.