Based on the phrase "change which charities I donate to" I had assumed he or she was already donating to multiple charities, presumably including action in subsaharan africa.

The money being donated to charities that are not in Sub-Saharan Africa would be better donated to charities that are. Even if that were not the case, that would just mean that the money that is donated to charities that are in Sub-Saharan Africa would be better donated to charities that are not. The money from a single donor isn't enough to change which continent you should donate to.

Also can you explain the "magnitude" thing?

An order of magnitude is a power of ten.

I'm not sure I follow your definition of "effectiveness".

Here's an example of what I mean.

The Seeing Eye trains dogs to help mitigate the effects of blindness for about $50,000 each. The Fred Hollows Foundation performs cataract surgeries to cure blindness for about 25$ each. It's not generally clear how to relate how much good two different charities are, but it is pretty obvious that a cataract surgery does more good than a guide dog, and for 2,000 times less. Thus, the Fred Hollows Foundation is more than three orders of magnitude more cost-effective than The Seeing Eye. Even if The Seeing Eye was tax-free and the Fred Hollows Foundation was taxed at 99.9%, it would be worth while to donate to The Seeing Eye.