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Where is the rule that says you measure from hull on vehicles and the flyer base on flyers?

It's on page 2 of the Battle Primer, in a side bar titled 'tools of war'.

Specifically you measure from hull when the model doesn't have a base. Flyers have bases so you measure from them.

Page 176, 'Tools of War'.

But keep an eye out for rules that override this rule and tell you to measure to the model instead of the base.