How I mark long sashing or borders
 

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?

I think that would work nicely. A friend of mine does something similar with blue tape and I filed it away in "good technique to use" mental file.

I really prefer to put my sashing on as an L whenever I can and avoid what I call "long unwieldy strips" but unfortunately some quilts want some things...

On my recent Scrap Metal quilt, I wanted to show off the subtle stripe of the fabric and so I cut long pieces the long way of the grain. Because I know I already trimmed my blocks to be precise, I prefer to not measure at this step but to go along with the existing conditions so I smooth everything out on my ironing board and pin the heck out of the sashing strip which is cut slightly long, I mean like every 1.5-2", I pin down low with long quilting pins so they are never under the actual foot nor near the needle. I do this for each column. Then when I put the columns together I actually layout it out by where the seams at the end of the row match up to the row below it. Unfortunately, I didn't take a process photo at that stage (but I thought about it), will have to next time it comes up. I am trying to write up a tutorial on how to figure out the measurements long pieces of sashing for diagonally set quilts and took pictures related to that. Thanks for the prod on my to-do list!

Great example.

That extra time and effort does pay off with better results. I also like to work in points/intersections so they match even it means adjusting a seam here and there.

PS - I lay the sashing or border on a flat surface before I start marking - to minimize stretching and/or distorttion.

I also lay the sewn strips/top that I want to sash/border on a flat surface before measuring.