Keeping integer addition within bounds
It will be a bit shorter and simpler without the duplicated else blocks:

if (plusZoomPercentage &gt; 0)
{
    /*Check for overflow or otherwise going out of bounds*/
    if (newZoomPercentage &gt; 100 || newZoomPercentage &lt; currentZoomPercentage)
    {
        newZoomPercentage = 100;
    }
}
else if (newZoomPercentage &gt; currentZoomPercentage)
{
    newZoomPercentage = 0;
}
zoomToPercentage(newZoomPercentage);
return AER_REQ_RESPONSE_READY;