+1 on the relay using next to no power, don't worry about the power going to your high beam.
I hope the ballasts are inside the light housings!
+1 also on the higher-spec cable. Voltage drop is proportional to length and is reduced by thickness of cable (you can't prevent it, or reverse it, you can only reduce it). Heavier cable also has less tendency to heat up as the current flows through it.
I do recommend a secondary switch for the lights though, don't run the relay directly from your high beam - run the relay power from the high beam wire through a switch inside the cabin and then back out to the relay. This way, you can hit high beams without the roof lights coming on - important when there are police around, because any white forward facing light higher than 1400mm off the ground is illegal, and you can only have 6 white forward facing lights - 4 on the roof gives you 2 too many.
So that wiring will go like this: pin 85 to ground. Pin 86 to a switch inside the cabin which comes back out to the high beam power out the front. Pin 30 to the battery with a fuse in the line (I recommend a 40A fuse for that). Pin 87 should go out to the lights. To make things easy (and because the relay power doesn't use a lot of current) you can just run a light pair of cables in through the firewall, connect both wires to the switch inside, and connect the other end as described (one to pin 86, one to the high beam wire).
Clear as mud?
