Keep in mind both pressures are measured via aneroid transducers, so they are absolute pressures. Thus when shutdown at SL, both are measuring 14.7 psia, and the ratio 14.7/14.7 = 1.00.
At takeoff, EPR is 1.65 to 1.8 or so depending on the engine. If the ratio reaches 1.89, the nozzle is sonic velocity (very very noisy!)