Well, EPR is a direct measure of thrust, N1 is not. To get a specific performance, we need to achieve a certain ammount of Thrust.
If you go the N1 route then somewhere in the process you need to account for variations in thrust from engine to engine for a given N1. Obviously poorer than normal performance is a problem, but also better than normal (because of minimum controlability). Obviously limits can be established with runs in a ground cell for confirmation of performance. The only engine I'm familiar with (the details of) works by a correction factor being calculated for each engine that accounts for how much more thrust is created than the minimum spec. On that basis a downwards correction factor is calculated.
e.g. if the engine can produce its rated thrust at 99.5% N1 instead of needing to be at 100%, then a 0.5% fiddle factor is sent to the thrust management system and the engine instruments. The result being that when engine is producing rated thrust and actually doing 99.5 percent both the N1 display and the autothrottle, by virtue of the 0.5% correction, think its doing 100%.
So you can see that getting the correct thrust developed by an N1 based engine requires some steps. Now, I'm not familair with the equivalent inner guts for an EPR based engine but would it not be a bit simpler as EPR is a direct thrust measurement?
pb