Accurate cerebral blood flow (CBF) quantification using arterial spin labelling (ASL) depends on physiological and MR parameters. Labelling efficiency is particularly relevant given it may vary between vascular disease patients. We determined subject-specific labelling efficiency values using phase-contrast MRI scans in a mild stroke cohort. Bland-Altman plots suggested a bias in CBF, with nominal labelling efficiency values underestimating at low and overestimating at high CBF. Using subject-specific, but not nominal, labelling efficiency showed plausible associations between white matter CBF and smoking status, pulse pressure, and age. Subject-specific labelling efficiencies appear to mitigate variance and improve CBF quantification in clinical ASL.