Outer radii of the hot inner flow obtained from the characteristic frequency of the fundamental QPO vs inner disc radii obtained from spectral fits. There is a tight correlation between these two radii for observations in which type-C QPOs are observed, which holds on timescales of hours as well as days. The dashed (blue) line indicates the best-fitting linear correlation using only observations in which the fundamental QPO is detected with more than 3σ (filled circles).