If you are getting no x-over at 50%, I would go with that.
The .5V at the cathodes is not 'going blindly' as you know the value of the cathode resistor (10 ohms). So it is actually sensing the cathode current, and asking you to set for 50mA per pair, or 25mA per tube. When you are given a voltage spec. at the grids, that is when you are going blindly and current can be all over the place.
In this case, at 700V plate, their spec works out to 70%. I prefer your 50%, and I think the tubes will too.