Large optical flats play a remarkable role in advanced large-aperture optical systems and the testing of the surface shape error is indispensable for the fabrication. The widely adopted Ritchey-Common test for large optical flats will fail without the rigorous test configurations including a large F/# prerequisition and a flat-to-interferometer distance invariance. A virtual-real combination Ritchey-Common interferometry is proposed to avoid the large F/# prerequisition by accurately modelling the optical path in a virtual interferometer. Furthermore, a virtual-real combination iterative algorithm is proposed in this method to break the flat-to-interferometer distance invariance. Measurement experiments for 100 mm and 422 mm aperture flats were performed to demonstrate the feasibility of this method. Compared with a direct testing in a standard Zygo interferometer, the peak to valley (PV) and root mean square (RMS) errors were less than 0.1 λ and 0.01 λ (λ=632.8 nm), respectively, in different Ritchey angles and flat-to-interferometer distances. Further numerical simulations demonstrate that RMS errors for various Zernike aberrations in arbitrary F/# are less than 0.01 λ. This method can break the distance invariance restriction and achieve high accuracy with an arbitrary F/#, thus providing substantial freedom in the design of test configurations to accommodate various test scenarios.