I think the issue with the math is the 50,000 number. What is your source for that? Because this says:
http://www.krgv.com/campaign-clinton-raised-over-45-million/33902614
That makes it sound like 50,000 was the target number of donations over a short interval of time, not the whole quarter. She would have to have well north of 100,000 donors total for the other numbers to work.
1) 91% of donations isn't the same thing as 91% of donors. A donor can make multiple donations.
2) The 50,000 donations number seems a bit low for the first quarter run of the Democratic front-runner. Especially if Sanders had 200,000. It's probably 500,000 donations, not 50,000. That figure could be of donors instead of donations, too. Media reporting is often sloppy.