bearisgray

I measure the length of what I want to sash or border - at least in two places - and then use an average of those measurements.

Then I cut border or sashing to that length plus st least 1/2 inch more (wiggle room - just in case)

Then I measure the intervals between the blocks and sashing strips on the sewn together unit.
Again - if the blocks/units are "supposed" to be the same size, if the variances are +/- 1/8 inch or from the "supposed to be" size - I will use the "supposed to be" size.

Same thing for the sashing strips.

Then I mark the long cut piece and draw a light line (whatever does not show through to the front and you can see - across the width of the strip - so that I can line up the second side of the strip so that the units will line up evenly with the first row/column.

This measuring and marking does take some extra time, but I've learned that my "eyeballing" did not work as well as actually measuring and marking the strips.

If this is not clear - maybe someone else can jump in and clarify?