Nelson makes them in any offset you want. He also has them in stock in very small increments (like 1/16" or 1/8"... something like that).

SWB's arms also fit the bill... just make sure if you decide to order an offset arm from SWB that you ask Scott what the design distance from the servo hub to the rudder hingeline is. As long as you can mount your servo close to that dimension you should have a good setup. As a note, the actual offset of SWBs arm are less than the formula gives, but the formula only takes a one dimensional correction into account... the SWB are also slightly shorter, which can also prevent slack. They give you the added benefit of SLIGHT amount of additional torque delivered to the rudder. The setup he used can only be calculated on CAD... I can't figure out a precise way to do it since the cable tension will change slightly through the servo travel... he has a good compromise setup.