Even though I knew the Air draft of my GB42 with the mast lowered back to a 45-degree angle, I always remeasured it before setting off on a long trip/cruise. I tape the end of my tape measure's tape to the end of an extendable pole of some sort and stick the pole up to the highest point on the boat. I take the tape to a nearby railing, note the reading and then use a level on a small board which extends outboard over the water. Then I run the end of the tape to touch the water and add the two measurements for total air draft.
BTW most trawlers have a pivot at the bottom of their masts. The trick is to install the hard points necessary to support the rigging necessary to safely lower the mast. I used to be able get mine down from 24 feet to 17 feet in 2.5 minutes. 17 feet was enough for all but the lowest bridges in Florida, Mississippi, Alabama and Tennessee.
Photo shows my mast in its normally lowered position under my boat shed. Blow it up to see some of the tackle.