Let me offer an alternative. First, your drawing imples the fence boards are a little less than 3" wide (i.e, 8-5 =3, but that's the hypotenuse of a 5° triangle). That's a odd size, so I assumed 3-1/2" (i.e, a 2x4). If the boards are really something else, I will redo the calculation.
1) Carefully unplug your miter saw and put it in storage.
2) Take your board and using your steel rule, make a mark
88.306" 88.669" from the left end on the edge nearest you. Call that the bottom edge. Draw a vertical line to the opposite (top) edge.
3) Measure exactly 0.306" in from the bottom edge on the left end, and 0.306" to the left on the top edge at the previously made vertical line at the other end.
4) Using your straight edge, connect the left end mark to the upper left corner of the board.
5) On the right end of the board, draw a line from the top mark to the bottom of the vertical on the same end.
6) Get a framing saw (hand is safer) and cut at the lines two angled lines you just drew.
You should now have your board with parallel ends at 5° to the cord that will span 88" horizontal (88.363 " on the angle)
EDIT: Misread the original drawing and corrected for the horizontal distance v. slant distance. Directions corrected.
Attachment:
