This is how i always understood it but there seems to be conflicting info. Even on this site it has an article taking into account resting heart rate, whilst all the others seem to use only MHR. The problem is the two different methods result in massive discrepancies in heart rate.
ie MHR:190 X 70% = 133BPM
MHR:190 - RHR:60 = WHR:130 X 70% = 91 + RHR:60 =151BPM
Where RHR is resting heart rate and WHR is working heart rate. Just using MHR is far easier but doesn't take into account the huge difference between beginners and elite athletes working heart range. Using the two methods causes so much confusion that experienced athletes think they can walk and still improve!! This article and anymore that talk of heart rate training must be more clear.