Source of magic balance cap assumption

From my own memory I wouldn't say it's 50%. Downward fluctuations in damage happen often, but it's definitely not half the time.

To answer your other question, I'm generally sporting 1k+ Int at any given time while not wearing anything, even a title.

If the occurrence of downward fluctuations is significantly less than half, that would suggest that the cap, if there even is one, is more than 100%.

Could you tell me how often you get less than your max dmg for a spell with a wide min-max range?