This paper presents a new method for calculating bistatic responses from retrodirective arrays when only monostatic measurement data is available. The proposed approach offers considerable simplification to the measurement facilities required for retrodirective array characterisation since bistatic measurements are significantly more complicated to perform in comparison to monostatic measurements. The method involves making some minor changes to the monostatic measurement configuration, enabling the capture of active element patterns with magnitude and phase data, and combining these with a new expression to obtain the bistatic calculation. Very close agreement has been obtained when compared to actual measured bistatic responses of a 10 element retrodirective patch array. In particular the predicted beam position from the new method is within 0.25 degrees at 40 degrees scan angle, compared to a disagreement of 1.5 degrees using previously reported methods.