Geometric dimensioning & tolerancing (GD&T) is an important step in product design and manufacturing. A good GD&T design is vital to ensure the quality and manufacturability of a product. So far, a datum-based model has been utilized to expedite the GD&T design. Even though the speed has been increased, the quality of the GD&T design is still largely dependent on designer's previous experience, such as the experience in part design, machining, and measuring. Without more detailed guidance, engineering novices are likely to make a variety of common mistakes while they use the datum-based model. To address this issue, the authors collected and analyzed the typical errors from the previous GD&T designs and set up a specification that instructs the GD&T designer what should be and should not be done in each layer of the datum-based model. This rule-based GD&T design specification was piloted in 2021. The result shows that the quality of the GD&T design has been considerably improved. The ruled-based specification introduced in this paper does not only help produce high-quality designs, which are required by front-end research, such as autonomous machining, but also signifies a step forward in achieving the full automation of the GD&T process.