*Cheap and dirty method*
Step 1:
Assume infinite bus on the HV side of the service transformer.
Step 2:
Divide rated current of the transformer (unlikely 600A, will probably be less) by the transformer impedance, example with rated current = 600A ,and impedance of 5.75%, therefore 600A/0.0575 = 10,435A
Step 3:
Determine what the rating of the equipment is, if it's higher then this, tell the inspector it's 10,435A, if not go to step 4. (Usually not required for small systems)
Step 4:
Determine impedance of the cable 100' of parallel 250KCMIL (using effect Z from Table 9) = (0.033 ohms/1000ft) x 100 ft = 0.0033 ohms and reduce the amount of fault current by combining in accordance with engineering principles. (You can do what is called the MVA method)
The outcome of which will give yo about 8.3KAIC at the panelboard
Of course this was all based on a lot of assumptions regarding your actual system.