Others might correct me, but my understanding is that the clearance should be at its maximum at low water, even though the tide still appears to be flowing out. I suspect that the surface waters might still be flowing out whilst the current starts to flow up river nearer the river bed, something like that anyway.
If you're worried about the air-draft under the bridges it's best to phone the Yarmouth yacht station and ask their advice as the actual time for low water on the day might be different from the published tide tables, as the tides are affected by things like atmospheric pressure. We were on the Broads during and after storm Ciara in February, and we noticed that the tides and river levels were all over the place for the few days after that. We had flood warnings for a couple of days, where the tide didn't seem to fall at all (this was in Brundall) and another day the levels dropped so much some boats were grounded on their moorings. It was quite an something for us, as we hadn't experienced that before.