Hi Debbie, welcome.

Let me suggest an alternate way to check frame square instead of laying on your back and trying to hold a tape measure.

Jack the car up, place your jackstands with the frame level to the floor. Use a plumb-bob on a string to drop from the frame to the floor and mark the floor. Measuring between dots you mark on the floor will be easier.

I'm sure Mr. Parmenter knows this, but I suspect that his advanced years have taken a toll on his ability to remember things like this and pass them on to others.