Something else to consider...
As Arouse1973 stated, marking two points, measuring and using the same two points will result in a resistor of that value. Adding additional card that are prepped the same way will be like adding resistors in parallel.
Measuring from multiple points on the same sheet of painted graphite will not result in a parallel resistance equation when you use all of the points at the same time, unless you have painted or cut lines into the card to electrically separate the path between the points you wish to use.
A picture is worth a thousand words, can you please upload a sketch, picture, render... etc?
Give us some idea of how these will be used and how you are measuring them with specifics as if you were instructing someone over the phone to do the measurements for you.
Edit:
http://en.wikipedia.org/wiki/Sheet_resistance
Because the the material you are working with is a non-uniform thickness, the examples here do not 100% apply, but even if it did this subject could get potentially complicated.
We await some additional details to help us determine what we are dealing with.