I’d like to use a 50R/50T plate beamsplitter at a 12° angle of incidence (AOI). Your plate beamsplitters are specified for 45° AOIs, how will they perform outside this range?

Usually, reflection/transmission curves shift and broaden asymmetrically when the Angle of Incidence (AOI) is changed. Beyond the stated specifications, R/T ratios decrease as AOI is decreased.