Since the graph paper idea did not work with your thinking style, maybe you are a visual person?
lay them on an old sheet in the positions you want. draw lines on the sheet where all the "units" (units including sashings) need to link up. measure the distances from the fish to the lines, add 1/2" for SA.
cut sashings of those measurements.
Do you know what you are using for sashing & borders?
I'm thinking river rock or sea plant fabric.
I hope you end up where you want to be! hugs!