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When fitting rigid gas permeable (RGP) contact lenses, the K values, which represent the curvature of the patient's cornea, are crucial in determining the appropriate base curve of the contact lens. The K value recorded as 43.50/44.50 indicates the curvature of the patient's cornea, with 43.50 being the flatter meridian and 44.50 being the steeper meridian.

To fit the contact lens "on K," the general practice is to select a base curve that approximates the steeper K value to ensure a proper fit and adequate corneal alignment. In this case, the base curve of the contact lens fit would typically be set to the flatter K value or slightly flatter. However, selecting a base curve that is equal to the flatter K value, which is 43.50, achieves the intended fit for the contact lens.

Choosing 43.50 for the base curve provides a starting point, ensuring that the lens adequately interacts with the corneal surface while allowing for proper movement and maintaining the integrity of the tear film beneath the lens. Thus, this choice reflects a common and practical approach in contact lens fitting.