If you can't find hexagon bar that gives an exact scale or drawing hexagon size you can still make your nuts as close to drawing as possible. It is not unusual to come across "hybrids".
Renault 5 twin choke carbs were retained by M8 studs, but with 12 mm A/F nuts rather than the 13 mm "norm", because of space considerations, as an instance. And I have mentioned elsewhere Whit form thread fasteners with A/F nuts or hexagons, in "mass production".
So there are precedents for "artistic licence" to use the nearest metric hexagon for a BA thread.
Howard