Height of Water in Mabul

During the Mabul the water was 15 amos (cubits) over the highest mountain. [Bereishis 7:20]. According to one opinion an amah is the equivalent of 18.9 inches (Frank, The Practical Talmud Dictionary).

Question: How high above the top of the mountain was the water in feet?

Answer: 15 x 18.9 inches = 283.5 inches

283.5 inches / 12 = 23.625 feet.