Left and Right Half Triangle Vents are measured the same as the Triangle.
Just as a half of the vent.

Specifying "left" means the upright center height is on the right.
With the base point pointing to the left.

And specify "right" for the upright height to be on the left and the base pointing to the right.

These are common to be against or beside a chimney.

The base measure inside the framing is the width.

The height is the upright side to the top point.
These equal the vent "opening size".
Do not measure the top angle. It has no value.

For "tip to tip" measuring, it is the same concept.
Exclude any flange on the vent and only measure across the bottom of the vent.

And again, the sides are the height.
"Tip to tip" means the vent will be the measures ordered.

Did we miss anything?