As a first approach I take would be to sum all the loads and calculate the drop as though they were at at the farthest end of the run. If the answer is an acceptable voltage drop, then you are done.
Then I would take your approach and see if the voltage drop was clearly below the maximum allowed. If it was near the max, I might just up the wire by a size (assuming conduit installation). The cost of the wire is swamped by the cost of the devices.
To correctly calculate the voltage drop you need to calculate the drop to each load in the circuit. Take the first load, calculate the drop using total current. Then use the voltage at this point and the current for all but the first for the next drop. Rinse, Repeat, Rinse.