Trying to build a circuit to boost 5 V DC to 250 V DC with fairly low current output
Where are you getting 260V from? The datasheet has multiple references to 260 mV as the reference voltage, but this has nothing to do with he output voltage limit.  There actually is no limit on the output voltage as far as I can tell, so long as you use components that have high enough ratings - namely the output capacitor, diode, and switching transistor.  All you need to do is divide the output voltage down to the reference voltage and get the compensation set up correctly.  

However, for generating very high voltages, a voltage multiplier of some sort may be preferable.  However, this may also experience droop in the same manner as the charge pump circuit.