N1 produces most of the thrust. If it's spinning at the correct speed, you'll have your thrust, regardless of pressure ratios, ice, whatever. EPR, on the other hand...
iEPR. Is that Apple pressure ratio?
Let me google that for you!
Integrated engine pressure ratio - Wikipedia, the free encyclopedia
"Most of the thrust" How much?
"correct speed" What is the correct speed?
If I set 95% N1 on my engine I only know the rotor is turning at 95%. Without knowledge of air density this information is meaningless.