Larry, back in the day when I did my apprenticeship, we used to set 12 volt regulators to 13.75 volts for lead acid batteries. Luckily, I’ve never had the misery of working on a 6 volt system, but logic would dictate you’re looking at 13.75/2 = 6.9 volts.
I’m guessing you know that dynos don’t charge at very low rpm, as there is little to excite the field. So you need to raise the rpm to a reasonable level when setting it up.
Good luck, Norman
Norman Anderson