Given that the reports from players go from 1st rare to 840 scrolls before they got a specific kage we can safely conclude that you truly have an honest 1 out of 8 chance of getting mei as a rare. It'sone of the few trustworthy random mechanics in place.

You have a 65% chance to get her in your first 8 rares, so with 480 scrolls. This is assuming the chance of pulling a ninja is not influenced by previous draws (basically you can draw the same rare you just drew before). It rises to almost 80% with 4 more pulls.

If you absolutely need her she sometimes appears in events for 12-15k coupons. Its the only way you are guaranteed of having her.