18 inches in from each edge of the island. This should distribute the light evenly.

Is that from center of pendant? Otherwise that makes it 10 inches apart from rim to rim. Am I looking at this wrong? Thanks

78-36=42, so even if that's the cords, and you have another 8 inches toward each other of shade, they're still 26 inches apart. sounds great. in the short direction, you center them, obviously.

Yes, that's from the center of the fixture box.