Multichannel transmit (Tx) arrays are essential for ultra-high fields. In the absence of commercial options, such arrays must be built in-house. Prior to human imaging, however, safe operations limits for specific absorption rate (SAR) must be established. This is done using EM simulations, the accuracy of which has to be verified by comparing measured and simulated data. Unfortunately, EM simulations may not fully represent experimental conditions, especially for complex transmitter designs. In this work, we propose a numerical approach to determine uncertainties encountered in EM modeling that allows the determination of safe operation limits despite EM modeling errors.