How to use a true bar?

@swagTrying to figure out how to accurately check for base flatness

Is there supposed to be a consistent line of light under the bar or no light at all for a flat base?

Depending on the True bar, many have a wider section or narrow section for testing. Narrow is less forgiving and may show light all the way across lightly but when you test with the thicker/wider side, there may be no light at all coming through.
Light coming through shows the sections that are lower, so light coming through the middle is “edge high” and light on the sides is “base high”. Here’s a link with pics.

https://www.slidewright.com/true-bars/