I don't think it really matters. Once you are on Nexium, you have blood levels stable already. Also Nexium is enteric coated, and does not break down until out of the stomach. If your acid is really low it may start to absorb there but not likely.

So if you are an hour late for the Nexium you will not miss much.
Once you get on a schedule that new time will become your dose time.

You might think about whether you really NEED Nexium so much.
It also impairs protein digestion and when your protein goes down, your blood carrier proteins become impaired. Many hormones are carried by these proteins, including your levothyroxine.