Compute the two separately, and add up the cumulative voltage drop. You can use the tables in the back of NEC, or one of the calculators online like this one:
http://www.csgnetwork.com/voltagedropcalc.htmlDo you mean 3-phase 120/208, or 1-phase 120/240?
FYI, the "half-length" the calculations ask for, in your case, would be 450' and 110', as the electricity has to take a return trip down back the same wire, so the actual electrical length required for calculating voltage drop would be twice that.