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Just want to make sure I have the rule right, as I've seen it played different ways.

Master of deceit warlord trait (Codex Space Marines) lets you redeploy after armies are set up.

Based on how I look at the rules, this occurs before the roll for first turn. I've seen folks use it after the first turn roll is made. Am I right in assuming it has to be used before the roll to go first? If it can be used after the roll to determine who goes first, can you supply the ruling/reasons for that to be the case? (Just so I can back myself up in the wild).

Thanks for your time folks!

It happens immediately before the Begin the Battle step, which is after determining turn order. From the Core Book Errata:

After Deployment: These rules, which also include wordings
such as ‘after both sides have deployed’, ‘after both sides have
finished deploying’ etc. happen in the ‘Resolve Pre-Battle
Abilities’ step. The Lord of Deceit Warlord Trait (see Codex:
Space Marines) is an example of just such a rule. After all of
these rules have been resolved, the players move onto the ‘Begin
the Battle’ step, and the first battle round begins (triggering any
rules that are used ‘when the first battle round begins’).