It finally dawned on me that I could easily do that and know my cadence after a couple rides with the new cyclocomputer. According to Sheldon Brown's online gear calculator, my conversion factor is 5x, meaning @16mph, cadence is 80; @20mph, cadence is 100rpm; @24mph, cadence is 120. Luckily for me I chose 46/18 gearing as it just happens to work out that it is exactly 5x.