X-ray scattering methods were applied to the study of thin mosaic ZnO layers deposited on c-Al 2 O 3 substrates using Pulsed Laser Deposition. High Resolution (HR) studies revealed two components in the ω scans (transverse scans) which were not resolved in conventional "open-detector" ω rocking curves: a narrow, resolution-limited, peak, characteristic of long-range correlation, and a broad peak, attributed to defect- related diffuse-scattering inducing a limited transverse structural correlation length. Thus, for such mosaic fi lms, the conventional ω rocking curve Full Width at Half Maximum linewidth was found to be ill-adapted as an overall fi gure-of-merit for the structural quality, in that the different contributions were not meaningfully represented. A "Williamson-Hall like" integral breadth (IB) metric for the HR (00.l) transverse-scans was thus developed as a reliable, fast, accurate and robust alternative to the rocking curve linewidth for routine non- destructive testing of such mosaic thin fi lms. For a typical ZnO/c-Al 2 O 3 fi lm, the IB method gave a limited structural correlation length of 110 nm±9 nm. The results are coherent with a thin fi lm containing mis fi t dislocations at the fi lm-substrate interface.