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The best way to figure out what a good percentage would be for you is to measure it. Also note that "max heart rate" should be computed using your resting rate as a base, not simply 220-(your age). Most folks "max HR" is a good deal higher than 220-(your age) yields. It is just that it is profoundly uncomfortable to find your measured (vs. computed) maximum rate.

You can start by going out to a track and using the tried and true (if not terribly accurate) "talk test". If you can converse normally without breathing through your mouth or otherwise sucking wind, you probably are recovered enough to start the next interval. Bring your HRM and note the rate where this happens. After a few laps at speed and recovery, take and average and call that your number. Be aware this number will most likely change as you get fitter and faster.

Also note the talk test has a corellary called the "singing test". If you find that you are recovered enough to sing, then you are running to slow and should have already started the next repeat.

regards
Dan

PS: If you are measuring, then they are not fartleks.....