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Gary 

Are you sure the schedule is looking at MHR% (Heart rate / Max Heart Rate * 100) not WHR% (working Heart Rate)?

Working Heart Rate is "Max Heart Rate - Resting Heart Rate" and Working Heart Rate % is ((Heart Rate - Resting Heart Rate / Max Heart Rate - Resting Heart Rate)*100)

Assuming your Max Heart Rate = 180, you are correct in that 162 is 90% of 180

but if look at WHR (assume your resting heart rate is entered as 62 and Max Heart rate is 180) your working heart rate is 116 (180-64), using this WHR a heart rate of 162 would be 84% ((162-64/180-62)*100)

 Clear as Mud?

Cheers