[Maarten]

It isn't about 'extremely precise voltage', but about making sure the current is somewhere around 100mA. I do agree that measuring across the lowest voltage heater will most of the times give a better indication than measuring across the entire string or another heater, but this is in no way guaranteed by the valve manufacturer.

I'll do some calculations just to get a feeling of the possible accuracy.

In any case, measuring the voltage across the series dropper and then calculating the current, is probably the easiest way in terms of accessibility so the discussion remains a bit theoretical.