Reading the magnetic state of antiferromagnetic (AFM) thin films is key for AFM spintronic devices. We investigate the underlying physics behind the spin Hall magnetoresistance (SMR) of bilayers of platinum and insulating AFM hematite ({\alpha}-Fe2O3) and report a record SMR efficiency of up to ~1%. To understand the SMR field dependence, we analyse the role of the imaginary Néel spin-mixing conductance and find it key for the SMR response of uncompensated AFM interfaces. We demonstrate that the combined real and imaginary conductances can be used as a unique tool to unambiguously determine AFM textures
