High-power radio frequency (RF) systems, such as those found in high-voltage/current test beds for RF component testing and RF plasma heating antennas, often experience standing waves (SWs). In such scenarios, the amplitude of electromagnetic (EM) fields ceases to be longitudinally homogeneous, and the resulting electric field gradient nonlinearly influences electron trajectories, introducing challenges in predicting multipactor, the exponential electron-growth mechanism, compared to traveling wave (TW) cases. This study identifies a specific regime where the mean and maximum electric field magnitudes characterize the upper and lower multipactor thresholds independently of the reflection coefficient. This unique regime enables the prediction of multipactor thresholds in devices using simulations with a single, forward-TW, eliminating the need for extensive simulations involving multiple waves’ excitation. Unlike previous works focusing solely on predicting thresholds initiating multipactor in geometries subject to SWs, our interest extends to predicting the upper multipactor thresholds, beyond which electron-growth diminishes. We use the commercial software Spark-3D, employed as a breakdown analysis tool, to determine the lower and upper multipactor scaling factors for complex 3-D geometries subject to SWs. By comparing multipactor electric fields for SW cases to those for TW cases, we propose multipactor electric field thresholds that remain constant independently of the reflection coefficient within the frequency range of interest.