I've seen somewhere that the recommended height is 10° below shoulder height. The rationale is obvious: it's so that if you roll upside down, your body does not sag as much, while the spine compression in the event of a frontal impact is minimal.
IVA does not have a lot to say about the position of seatbelt mounts, just that they have to be 45 cm above the reference brick on the seat (IVA 31.16). The 550 mm high chassis seatback rail is high enough only for very low seats, most seats with runners cannot be so low.
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