Divinehover: EPR had one quite useful property as I recall from the B747-200 with RB211 engines - since the EPR increment over 1 related to the thrust produced then, say, if approach power required was say 1.25 EPR then this gives a thrust equivalent to + 1.0 EPR ( 0.25 x four engines) therefore on three engines the EPR required would be 1.33EPR ( +1 delivered by three engines) Similarily for a two engine approach the EPR would have to be 1.50EPR on the two running engines. This relationship does not hold for N1s. Other aircraft use total fuel flow as a similar approximation since thrust relates to fuel flow.
An EPR reading of less than 1.0 for instance in the descent is merely confirming that the combination of fan + core is actually producing drag!
Hope this is helpful to you?