1. A method for adjusting operation of a speech recognition engine implemented as a plurality of computer processes functioning in a computer processor and characterized by an associated receiver operating characteristic (ROC) curve, the method comprising: in a first computer process, interpreting user confirmation of speech recognition results within a given confidence score threshold to create a confirmed portion of the ROC curve for the speech recognition engine; in a second computer process, extending the confirmed portion of the ROC curve by extrapolation of unconfirmed speech recognition results beyond the confidence score threshold to generate an extended ROC curve; and in a third computer process, adjusting the confidence score threshold based on the extended ROC curve to meet target operating constraints for operating the speech recognition engine to perform automatic speech recognition of user speech inputs.