http://www.georgebelton.com/E85Calculator.php
Maybe 2 gallons at most per tank full. 2 gallons will raise the octane a little over 1.5 points. If your 93 fuel is more like 91, this should clean it up. You could also run a 91 octane MHD map with your 93 gasoline to see if it cleans up - that may be the conservative way, and might not reduce performance much at all if you are getting pulls on 93, and also might feel smoother without the pulls.