How do you measure between two internal surfaces accurately?

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Measuring between two internal faces usually means measuring around whatever is in the way and adding the difference back, which is a guess dressed as a dimension. Scribing both surfaces, then aligning the T1 Tomahawk's tip to one line and the heel to the other, gives the distance between the faces themselves. The number on screen is the one you cut to.

How to measure between, not around

Scribe both internal surfaces first
Switch the T1 to measure from the back
Tip to the front line, heel to the rear
The dimension is between the faces
No adding a difference back afterwards

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Scribe both ends first, then measure to your own lines. Mark each internal surface with a pencil, clear the obstructions, and switch the tape to measure from the back. Align the tip to the front line and the yellow heel exactly to the rear line, and the dimension on screen is between the two surfaces, not around them.