X-ray scattering methods were applied to the study of thin mosaic ZnO layers deposited on c-Al2O3 substrates using Pulsed Laser Deposition. High Resolution (HR) studies revealed two components in the omega scans (transverse scans) which were not resolved in conventional "open-detector" omega 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 films, the conventional omega rocking curve Full Width at Half Maximum linewidth was found to be ill-adapted as an overall figure-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 (00.1) 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 films. For a typical ZnO/c-Al2O3 film, the IB method gave a limited structural correlation length of 110 nm +/- 9 nm. The results are coherent with a thin film containing misfit dislocations at the film-substrate interface. (C) 2011 Elsevier B.V. All rights reserved.