I looked at various calculation methods and online calculators. I have found this chart to match very well with perceived exertion levels when I'm riding:
http://en.wikipedia.org/wiki/Image:Exercise_zones.png
I just watch my Garmin 705 and upshift/downshift as necessary to keep my cadence just over 100rpm and my HR at the border of the Aerobic/Anaerobic zones for my age.