How do you calculate the voltage drop for each fixture in a site lighting circuit?

Due to the various lengths between each fixture, it will likely make your head hurt.
Try this:
Mike Holt
The scroll to bottom to where it says "Voltage Drop Calculations"
I would cheat on this and hook up good Voltmeter, and screw in or uncrew each bulb one by one and take readings? Good Luck

If you are designing the circuits and feeders, the simplest to do is sketch it all out on paper, laying out your circuits the shortest and/or reasonable method, and mark each light location with the total distance back to the distribution point and the total load in amps for that circuit.

The load will reduce by one fixture but longer distance as you work you way farther from distribution.

Start with the first fixture on the circuit, total the load and measure the distance and perform the voltage drop calcs at that point to size the feeder.

Add the distance to the next load and deduct the first load, and do the calcs again, and again, for each difference in circuit data, until your done.

If the variables are great, you may notice the wire size reduce quickly as you get further out. Don't reduce too quickly as to allow for future growth.

I told you your head would hurt.