Okay I'm up on the ladder...I've got the tape butted in the corner...what do you read on your end?
I'm guessing you're trying to measure in between two inside corners? If so, butt the tape in one corner. It helps to have a helper hold it there. Then stretch it tightly to the other corner. Hold the casing (housing) in your hand and "roll" metal tape into the corner, you'll get a decent measurement that way.
Method two...make a pencil mark on the wall about halfway between the two corners. Measure from one corner to the mark. Then measure from the other corner to he mark. Add the two.
Another option: The tape casing does have a defined size, it's usually marked on the side of the casing...somewhere around 2.25-2.3 inches, though it depends on your model. Supposedly, you can put the tape hook into one corner, and slide the tape casing into the other corner. Take the measurement off the tape right where it enters the casing, then add the size of the casing to that. I never use that method.
Remember, it's always better to cut the piece slightly long and have to trim a sixteenth or an eighth, than to try to cut it perfectly on cut #1 and come up a tad short.
The trim piece should actually be about a sixteenth long so the board can then be flexed and "snapped" in to place.
I hope this answers your question...though you never really asked one.