I don't know if I have the experience needed to make this reply, but the best advice I have heard (and followed) on cadence was this -

If you are breathing easy and your legs are dead - your cadence is to low.

If you can't get your breath but your legs feel fine - your cadence is to high.

if both legs and lungs are equally tired - your cadence is fine, adjust if you want to, but don't feel that you have to.

(for those who take everything litterally - these examples are the extremes. Use the principal to adjust cadence up or down until your lung power and leg power run out at about the same time)

Michael