A tartan is defined by the width and sequence of stripes, called the sett. They are defined and registered through a threadcount, which is the formula for weaving the plaid. The fabric you see is the result of applying the formula to weaving.
Threadcounts are never used to create the needlework charts. I look at a good picture of the tartan. These must have colors that are distinct in the picture and be flat.
From this picture I find the narrowest stripe. Looking at the picture I then calculate the widths of the other stripes. Once I have this figured out, I then calculate how wide it is to see if it is suited for counted work and make adjustments.
Once I have a formula, which may take several days to create, I begin to chart the plaid using a needlework charting program. Each direction is charted separately and in color. A third chart of the completed plaid is created, and is checked to see if it looks correct.
Corrections are made if needed and the charts are put into the template for the technique of your choice. Corrections and additions (sometimes quite extensive) are made so that your plaid chart is perfect for your thread choice and stitched object.
Every plaid I chart presents its own challenges. Although this is the process I follow no matter the type of plaid, each one is an original charted adaptation of a plaid.