Follow-up: Got it all tightened up this afternoon with a little help from my wife. Loosened the front brackets, which I kept on top of the rubber channel guards.. Loosened the center mounts so the rack would flex upwards. Pulled each side upwards at the front until we could get a finger underneath it, then marked the front bracket. Same on the other side, then removed the front bracket and tightened each side at the marks. Reinstalled the front, lifting the sides as required. Tightened the center mounts, which put pressure on the front and got it very tight. I now see why the front mounts are two piece construction - it lets it get tighter, with full contact against the sloping forward section of the roof.
Worked far better with a partner rather than solo in the driveway racing against a setting sun.
Very happy overall.