The grade I'd give a US President isn't fixed over time.
A good example is actually the first Bush Administration: six months in I gave him a D, which I thought to be a well-established and potentially stable mark. The policies were dumb and wrong-headed, but at least he hadn't gotten us into a war yet.
His grade probably would've peaked at a C- about nine months later, after he recovered from the jolt on September 11 with the relatively successful war by proxy in Afghanistan and before his folks' ambitions for aggression in Iraq were nakedly revealed. It's an F, of course, from then on, until such point as we kick him out of the classroom when it becomes apparent the Iraq whole thing was planned to be a stage-managed photo op to make him look good in his re-election campaign.
Botched, like everything else, Bushite Misrule showed that two negatives--bad policies, badly executed--don't make a positive. Just chaos.
Still, Obama represents such a pivot toward the light that the range of grades I'd consider would only go from A+ to C. I think a solid B is tough but fair, as I'll explain over the three critical dimensions: economic, diplomatic, and political.