Set the angle of the top panel with the shims, see below. Lock them in place with a wood screw thru the shim lock then down on to the top panel thru the 1/4" slot into the shim.
Similarly, insert a shim lock on the back side of the top panel, screw up thru the slot into the fence.
Place shims 3" to 6" apart to prevent deflection. Use double sided tape to secure a shim at the very end of the top panel.
Set Taper Angle:
Setting the taper angle couldn't be simpler. Layout the angle on one of the leg blanks. Set the jig on your bench top with the blank adjacent to it.
Insert 1 shim and slide it back & forth until the edge of the apron is lined up with the layout line on the blank.
If necessary, place a flat board under the blank to raise it up to the apron height. Also, slide the blank back or forth as desired to adjust to the apron's height.
The location of the blank does not matter. It just needs to be somewhere along the side of the jig.