Just to clarify, I do not recommend marking the average width of the strips. I recommend marking the actual seams of the strips.

The averaging is when measuring the width of the pieced blocks, so the sashing can be cut that width. That is actually the first step. Above is the second step.

This is the same method I use when using blocks that are going to be sashed without cornerstones. It keeps all the blocks lined up with one another.