A fundamental concept about diamonds is illustrated by this quote, "A diamond's proportions, particularly the depth compared to the diameter, and the diameter of the table (the largest and topmost facet of the diamond) determine how well light will travel within the diamond and back to the eye." from the Jewelry.com Web site article: 'Understanding Diamond Cut.'
This guide teaches what diamond table and depth proportions are, how to locate and define them, and how to calculate their proportions for determining the quality of the diamond has cut (one of "The Four C's" of diamond quality that determine diamond value).
Things You'll Need
 Reasonable math skills or a hand held calculator
 The diamond's Table measurement in millimeters (this information is exists in the gemological certificate for the diamond in question).
 The diamond's Depth measurement in millimeters (found on the stone certificate cited above).
 A scratch pad
 Jewelers matrices used for determine diamond Cut Grades based in part on Table and Depth dimension relationships.
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Understanding diamond cut, table diameter, and stone depth

Briefly study the graphic in this step in order to help visualize a diamond's Table and Depth in relation to a diamond's overall cut. Note the diamond pictured at the top illustrates what parts of the diamond are included when measuring depth, while the diamond beneath it depicts the stone's table measurement area. Note the line drawings in this graphic to the left and right that show the results of too deep and narrow, or too broad and shallow cuts.

Calculate a diamond's depth percentage. Different diamond cuts require different calculations, the results of which will categorize the diamonds ability to reflect light and disperse it. Using a gemological report for the diamond in question, perform one of the following two calculations.
To calculate the depth percentage of a round brilliant diamond, use this equation: Depth (in mm) divided by average girdle diameter. (Note the girdle is the widest part of a round diamond, and may have thin, medium, or thick dimensions itself).
To calculate the depth of all other "fancy" cut diamonds, use this equation: Depth (in mm) divided by total width (in mm). Depending on the cut of the stone, depth and width calculation results of fancy cut diamonds vary greatly from standard round cut diamonds.
Jot down the answer to your calculation, with a notation that this is the depth percentage of the stone.

Using a depth percentage matrix that grades diamonds from "Ideal" to "Very Good" to "Good" or to "Poor" in depth percentage, find the percentage range of the calculation fits on the jeweler's depth percentage, find the percentage range that most closely aligns with the answer to your calculation, based on the cut of the stone in study. (Reference 2)
The Jewelry.com Web site provides such a table, and the hyperlink in this article titled Depth Percentage for Different Diamond Shapes is the type of reference matrix required to grade diamond depth.

Now calculate the diamond's table proportion. (Reference 3)
To do this for a round brilliant diamond, divide the longest table measurement (in mm) by the average girdle diameter (in mm).
Jot down the result. (Typically, "fancy" cut stones are measured that type of stone cut by its smallest diameter.)

Using an Ideal Table Proportions matrix, which grades diamonds on a scale from "Premium" to "Ideal" to "Excellent", find the percentage range in the matrix within one of these grade categories that most closely aligns with the calculation results.
The Jewelry.com Web site provides such a table, and the hyperlink in this article titled Ideal Table Proportions of Different Round Diamonds links to the type of reference matrix required to grade a diamond's table proportion.
Tips & Warnings
 The results of the diamond Depth and Table element calculations, when applied to their respective standard grading matrices, substantiate a professional gemologist's certification and grading of a diamond in one category over another. The matrices provide professional standards to which gemological society professionals adhere. Therefore, a gemologist might further categorize a diamond as "Fine Cut", where the maximum amount of light both enters and is reflected back from the stone, making it appear larger than a stone of the same size but of a lesser grade and quality. These diamonds acquire a "Shallow Cut" grade. Light enters the diamond but does not reflect from it, or "Heavy or Deep Cut". In this later case, light enters the stone, but leaks out the sides rather than the top of it, and these stones often look grayish or darker than other, better cut stones. (Reference 3)
 This guide is informational in nature and does not attempt to teach professional gemology. When calculating the table and depth of a diamond, do not become over critical of seemingly lesser quality stones. Many stones with lower grades are still very good diamonds costing several thousand dollars each for a one carat stone. Highly graded stones are quite rare, but when identified, they attain an almost museum piece stature, and are sold in the hundreds of thousands of dollars range.
References
 Photo Credit ~graphic designed by the author of this article