How do you calculate the optimal cruising speed of an airfoil?
The cruising speed is only one parameter. For optimum flow conditions, you need to look at the Reynolds number and the lift coefficient at which the airfoil shows its best behavior and then tailor the aircraft to achieve those numbers at its design operating speed. Normally, a higher Reynolds number is better, but at low speed (= low Reynolds numbers) airfoil performance drops off a cliff. It is important to stay above this limit in order to get good performance.
This tailoring is done by selecting the wing area in order to achieve the right wing loading (the ratio of airplane mass to wing area) and the wing's aspect ratio (that is the ratio between span and chord) to arrive at the lowest possible drag. Selecting too high an aspect ratio (wing becomes too slender) will drive up the mass of the wing structure and reduce its Reynolds number. Conversely, picking too little wing span will give you plenty of wing chord, but induced drag will be higher than necessary.