Do a search for "pencil mod." It works on both the red fury and blue fury models. I have two red furys running at ~2.8-2.9 GH/s. You can tune it by playing with how much graphite you lay down with the pencil.
Or if you're feeling brave you can connect the two points used in the 'pencil mod' with a 100k pot and slowly reduce the resistance whilst measuring the ASIC voltage.