This may just be one of those instances where left of the Atlantic people use different terminology than those on the right. The pinch rod I am referring to is basically two sliding lengths of timber that can be extended to the full height or width of an opening and locked, so a measurement can be taken. Here is one I made for ceilings – when both pieces are closed it measures precisely 2000 mm. By extending the sliding piece to the height of the ceiling, I simply have to measure the length of extension and add that to 2000mm to get an exact ceiling height.
Doing the best I can with what I've got