I know somebody who does regular RAMP tests and uses that to get his max HR (as well as testing his wattage).
After a brief warm up on a turbo, he starts off at an easy endurance pace and then ramps his power up by 20w every 1 minute (or some other incremental increase if you haven't got power to work with), and continues this until he can no longer keep the power up (usually takes him around 15 mins to build to this point), and then warms down gradually for 20 mins.
He takes the max observed HR and adds 3bpm to estimate his theoretical absolute max. (Not sure why 3bpm exactly, apart from it being a "little more, but not a lot".)
For his HR zones, he applies the following %s to his theoretical absolute max HR...
Recovery - under 65%
Endurance - 65% - 77%
Tempo - 77% - 87%
LT - 87-92%
Not sure how valid this is, I'm sure it has it's pros and cons, but it does seem to tally nicely with my HR (I see in zones I set via power).
Anyway, hope it may be of some use to you
Cheers, Gaz
EDIT - I forgot to mention that these zones seemed quite wide to me, eg around 20bpm for me for endurance and tempo, so you may want to aim for somewhere in the middle, or even split the wide bands into a lower and an upper section.