I'm sure a computer would be more accurate and could be helpful. If I was a racer, I would get one. With me, 110-115 is a natural cadence, one that I am happy with. I only started timing my cadence to see what I was doing. It's pretty flat here in Cleveland and every time I counted my cadence I was always within the 110-120 range.

But I could see a cadence computer being very helpful, especially for newer riders trying to find their sweet spot. All I was saying is that if the OP didn't have one there are other ways to get a rough idea of the cadence.