This is what we used. DCDoug and I had a thread just recently about renting corner weight scales. But almost every local Cobra Club has somebody that actually owns a set (and you only need them for the afternoon). Fill your tank up about half-way, put something in the driver's seat that weighs what you do, then adjust to try and get a 50/50 cross-weight. Don't forget to loosen your sway bars first -- the pre-load will skew your numbers.
