The current OCP GeomLib_Tool.Parameter_s function stores the computed result in the 4th reference argument U and returns a bool indicating success.
https://github.com/CadQuery/OCP/blob/036b08c136528fecb6df570453be218df0ed72f6/opencascade/GeomLib_Tool.hxx#L51
Return value should probably be Tuple[bool, float] or something along those lines.