Humminbitd Helix9 transducer setup

When setting up what a unit what is the best way to measure the transducer distance so it will show the correct depth reading.

It depends on when you want true water depth instead of the distance from the transducer to the bottom and where. Measure from the bottom of the transducer to the water line and add that distance to the depth offset.

If the transducer is used for on-plane depth, you don't do anything since it is at the water's surface.

I would typically want true water depth in the front of the boat. So add that to the offset in the unit correcy.

Yep.
Setup menu/User Mode-Advanced/Depth Offset