Will work fine...
On the 3 tab horn relay, I think the power (30) and switching power (86) are the same. The output (87) is the horns, and the switching ground (85) is the horn switch. This is how I would do it, and is how it's done when the ground is switched, as in many stock circuits.
Check to make sure the gray black is switching to ground and is not hot.
Sometimes, like for fog lights where they should only work when the low beams are on, the (86) is connected to the lowbeams, so that the relay is dormant when the lights are off or when the highbeams are on.