When you enter ventilation and/or exhaust, the program's calculations may adjust the rate of infiltration to be more or less than the value you entered. If exhaust exceeds ventilation the infiltration rate will increase, due to negative pressure. If ventilation exceeds exhaust, the infiltration rate will go down, due to positive pressure.
CFMimb = ECFM - VCFM
ICFM = (CFMinf ^ 2 ± CFMimb ^ 2) ^ 0.5
CFMimb = CFM imbalance. Amount exhaust exceeds ventilation. Negative if ventilation exceeds exhaust.
ECFM = Entered exhaust CFM
VCFM = Entered ventilation CFM
ICFM = Final infiltration CFM
CFMinf = Entered infiltration CFM