The height of a bridal chair is decided by a camera, not by a sitting position. The seat height is comfort; the back height is composition.
A photographer framing a seated couple wants the crest of the chair inside the shot, with a margin of upholstery visible above their heads so the chair frames them rather than being cropped off behind them. Get it wrong by a few inches and the crest sits exactly level with the top of a headdress, which produces the one photograph nobody can retake.
So we ask for the sitters' seated heights, or a photograph of them seated with a known measurement in frame, and we add the headdress or turban height on top of that. The crest is then set above the total, not above the head. Where a dais is involved we ask its height too, because a chair viewed from below needs more back above the sitter than one viewed level.
Where those figures are not available, we work from the venue's stage dimensions and the photographer's likely position and tell the client the assumption we have made, in writing, before we cut anything.