Workload assessment is required for human factors validation of control rooms. The NASA Task Load Index (TLX) is a popular measure for this purpose. However, acceptability of workload is difficult to interpret based on the scores, and the measure is time consuming for use in test scenario breaks. This paper reports the identification and evaluation of a one-item subjective rating for the acceptability of mental workload. Study 1 found that one item, mental demand, highly represented the multidimensional TLX and that operators rated this item to be distinct from performance and situation awareness. Study 2 investigated the acceptance scale's sensitivity to control room modification, and the study found that operators' rating of level of mental demand and rating of acceptability of mental demand were moderately related. The rating of acceptability, compared with the original scale, was more sensitive to control room modernisation and was more strongly related to subjective performance. Operators' rating of acceptable workload was significantly related to observers' assessment of team performance, while the rating of level was not. Both acceptable and non-acceptable workload were related to high levels of the original TLX scales, indicating that operators' perception of acceptable workload was not fixated on a particular level of the original TLX scale. The results supported that efficient assessment can be achieved by applying only the TLX mental demand item and by supplementing or replacing the original rating scale with an acceptability rating scale. The approach presented applies to establishing ‘redlines’ for human performance constructs beyond mental workload. Relevance to industryHigh-consequence industries are required to perform human factors validation for new and substantially modified design. While numerous human factors measures are available, approaches for establishing cut-off scores or ‘redlines’ are lacking. This paper investigates acceptability rating to address this challenge.