To get conductivity, the graphite particles have to touch each other. The latex matrix will hold them apart, so you'll want a fairly high concentration of graphite. This Instructable uses a 3:2 graphite-to-matrix ratio: http://www.instructables.com/id/Conduct ... nd-Conduc/
To measure the resistance of the final product, tape out a 1" square and run copper wires across two opposing edges. Paint the square in a way that makes contact with the whole length of each wire, then measure the resistance between the wires.
Resistance is a material property, so in practice it depends on the geometry of the object. One side-effect is that resistance is independent of scale.. the resistance between opposite sides of a square will be the same no matter how large the square is. That follows from the basic resistor equations: putting two 1 ohm resistors in series gives you a resistance of 2 ohms. Putting two 2 ohm resistors in parallel gives you 1 ohm. If each 1 ohm resistor was a square of material, a 2x2 pattern of such squares will still have an edge-to-edge resistance of 1 ohm.
If you use fairly large copper wire (12 gauge would be good if you have some sitting around), the resistance of the wire will be a few orders of magnitude lower than the resistance across the square, so it won't even count as a rounding error in your readings.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.