the xp you gain now depends on your inscription lvl and the spell you try to inscribe.
i would suggest to add a factor: the lvl of the scroll you try to rewrite
example:
assume you are lvl 100 inscription, you write a spell and gain 100 (index) xp each successful attempt (now)
my idea would be: you gain 100 xp for a successful rewrite of a lvl 50 scroll (and below) but more xp for successful rewriting a higher lvl scroll, for you should gain more xp for harder things
no idea how this would affect game play, but how about making it 100 + min(0, (2*scroll lvl - inscription lvl)/4) xp?
thus you would gain 125 xp if you successfully overwrite a scroll with a lvl that equals your inscription lvl
