Howdy. I did this myself with a race bike a few years ago. At the time I was trying to raise the rear end to give me a steeper swing arm angle and better race geometry for quick turn in. I wish I had written down the ratio but what I do remember is that it only took a couple of mm to make quite a difference. If you have access to some steel flat and a drill press then I'd experiment a bit. I'd start off with 4mm length difference and then go from there.