If they ask, I tell them "both feet", then as long as they make it look like they're trying and they're not right next to the ruck area, whatever. (Don't think this year's referee policy says anything about it.) The one to watch for is the sneaky bugger at 5m and in who puts both feet on the line and then crouches into a runner's stance at the side of the ruck with his hands on the ground and inside the field of play, who both cuts off the scoot and gets a head start to fire out and pressure the dummy-half.