I did not re-add the measurements, but I would think that what ever size the pieced section is, that is what size you cut the plain strip to add. Unless you want a specific size quilt block. I have seen this tube made with three strips and two strips.
Two cut at 2.5 each would give you a 4 .5 inch plain strip. three stips would give you a plain strip cut at 6.5 inch.
With out making one, I can not say what the size of the block would be. well......
I did some cutting of paper and I came out with a 4.5 inch strip would yeild about a 6 1/2 inch, unfinished block, 6 inch finished. SO a 6 1/2 inch stips would yeild a 9 1/2 inch unfinished block, or 9 inch finished.
So it would depend on if you are set on a certain size block. If not then work with what ever size you want and just make your plain strip the same width and using the 45 degree angle, it will come out all the same..