I prefer to use the simple method for a few reasons, but in saying that, I certainly do acknowlege the variations to the simple equation that exist.

My personal Target Zone at 65% according to the Karvonen Method:
RHR 60min^-1
Age 31years

= (((220-31)-60)*0.65 + 60)
= (129 *.65) + 60
= 83.85 + 60
= 143bpm at 65%
As opposed to 123bpm without taking RHR into account.

But just say if my RHR is 90 - high Resting heart rate
RHR 90min^-1
Age 30years

= ((220-31) - 90) * .65) + 90
= 99 * .65 + 90
= 154bpm

I have calculated my Target Heart Rate at 65% for 2 different RHRs 60 and 90 (as demonstrated above).

You will notice that the Target Heart Rate for the RHR 90 is larger than the the one for RHR60. Ordinarily, I wouldn’t request a client with a lower RHR to work less harder than a person with higher RHR (but there are several other factors that must be considered by an instructor when prescribing an exercise program).

Usually, a person with a higher RHR will feel the increased exercise activity a lot more than a person with lower RHR, but, as aforementioned, there are many factors to consider and this may not necessarily be the case.

Mark is correct, and there are additional methods of calculating Target Training levels. The main thing is, when you exercise, especially if you are just starting out, make sure you are not in pain when doing so. Exercise should be enjoyable and fun :)