I have been making string quilts and just mark my square ruler with the size triangle they need to be cut.
Measure your tube of 4- 2-1/2" strips from bottom up to the top seam. Mark the corner w/painter's tape. Cut each side, flip over tube, cut both sides and continue onto the end of tube.
Now I am using 2" strips - 4 of them - measured, I marked under the 3-1/2" across the corner, cut like above, when four pieces are sewn together you will have 7-1/2" unfinished square.