I am performing arc flash calculations based on the IEEE formula's and I have a few cases where the calculated arcing current falls within the vertical instantaneous band of a molded case breaker's time current curve. Do I use the instantaneous time or go all the way up to the overload portion of the curve?? I assume worst case would be to use the longer time of the overload curve?

Arcing Current in the Instantaneous

Your assumption is correct. For example if a breaker's instantaneous band goes from 900 Amps to 1200 Amps, it could trip instantaneously for currents begining anywhere from 900 to 1200. Thinking worst case you would use the 1200 Amps for the inst. This means currents of 1000, 1100 etc. may or may not trip in the instantaneous and you use the overload part of the curve.