It is a good idea to start with a piece of grid paper and draw your circuit to scale. The bigger your piece of paper, the easier it is to work with. Computer people can, of course, do the same thing with almost any graphics package (like MS PowerPoint). Start with the full room size, as placement of your table is very important (see below). Do a rough design, lightly in pencil and then use a compass to start drawing your curves accurately, to scale. Just as you will do when you begin with the router, draw the curves first and then connect them up with the straight-aways.
 

Standard measurements (1/32 scale) for planning:

 
Slot width: 1/8 inch
Slot depth: as deep as your bit allows (usually 1/4 inch)
Distance between slots 3.25 to 4.0 inches
Distance from outer slot to track edge: 4.0 inches (be generous)
Distance from inner slot to track edge: 2.0 to 3.0 inches
Smallest practical radius: 4.0 inches (this is very tight!)
Largest practical radius: unlimited
Security tape width: 3/8 inches
  

 If you are running Scalextric "360" Porsches, you should make the outer track edge even wider to allow for the full rotation of the car in the slot. Be generous with the swing room.
 

 When I built my track, I drew some full-scale corners on paper, photocopied them and spread them around on the tabletop before I started construction. This gave me an exact visualization of the final version and allowed me to do some last minute tweaking of the design.

  And as my father taught me and your father, no doubt, taught you...."Measure Twice...Cut Once"