When cutting the panels and when routing the dados and rabbits, I found the E. Emerson Tool Co. C50 50-Inch All-In-One Contractor Straight Edge Clamping Tool Guide to be essential.  

Take the time to layout the panels before cutting on the 4'x 8' plywood sheet.  Be sure to allow for the kerf (saw blade cut thickness), between panels.

Before cutting the panels, cut 2 spacer boards about 36" long and:

• One board will be the width of the distance from the edge of your saw to the cut made by the blade. Using a sharpie label this board "saw".
• The other board will be the width of the distance from the edge or your router to the center of the cut made by the router bit. Label this board "router"

When laying out the cut lines mark on the exact distance, then place the appropriate spacer board on the line and butt the clamp up to the spacer block.  Clamp, remove spacer block and start cutting or routing.

This will save a lot of time and miscalculation of trying to add or subtract the distance of the spacer on every line you mark.  Note:  make sure you are cutting on the same side of the mark for matching pieces.