All offshore helidecks that I've been to have also had an HDO, or someone in overall control who always maintains eye contact with the driver. Rather than concentrate on the fueler at the nozzle, what about controlling the shut off via the HDO and/or the one at the pump? When the gauge is nearly full, indicate to the HDO or the pump guy, then they can reduce or shut off fuel flow there. If the tank needs further topping off, then the nozzle guy will be alerted, and reduce the fuel flow plus pay attention ;)

Our hot refuelling SOP's state that the chap at the nozzle must be in comms with pilot. Simple to do, safe and effective. Pilot gives running commentary about fuel state (200, 100 to go etc) and we normally stop just short of max fuel (but could go to max, just never really need to for our op stops).
Otherwise, control it via handsignals through the deck controller, who then tells the pump turner offer to stop it. Giving an amount works well too but just have to get it right or they might just keep pumping it onto the deck (seen that happen as the refueller was concentrating on the amount and not watching the fuel piss everywhere:ugh:)
:)