When filling kegs (with Starsan or beer) I use the overflow method. Fill through the liquid-out and vent through the gas-in port. As soon as I see liquid coming out the gas post, I disconnect it from the keg. (The liquid in the bucket is Starsan that overflowed when I pumped it out of the keg earlier).
@Brew Cat, how does your method save CO2? The way I understand it you would need to either keep the CO2 attached (as I do) or pressurize the fermentor so that you have enough pressure in the fermenter to finish the transfer before fermentor pressure drops below keg pressure, which would stop the transfer.
I've also seen a process where you have beer flowing from fermenter to keg through a liquid-to-liquid jumper and gas flowing from the keg back to the fermenter through a gas-to-gas jumper I have been thinking about this method too.