How far can I run a DC line to some LEDs
At 25A (300W @ 12V), the wiring is going to have to be fairly hefty.

Since the power supply does not have remote sensing, you'll just have to manually crank it up to compensate for the voltage drop in the wiring.

According to the datasheet, it can be set up to 13.2 V, which gives you 1.2 V of "headroom" for the wiring.

1.2V/25A = 48 m&Omega; total resistance.

2 strands of 40' = 80' total wire, so the wire can have a maximum of
48 m&Omega;/80' = 0.6 m&Omega;/ft of resistance.

Using copper wire, you'd have to use AWG8 to get to this level and deliver 12V to the fixture.