The so called "standard" Wolff also is meant to be used as a direct drop in replacement for an older gun with a tired spring. Or one that had a spring altered in the field. As was mentioned above, due to their different design the spring can be slightly lighter, while delivering the same hammer velocity. Your 66 is more likely to have digestive issues if it's a frame mounted pin dash# when using a light weight. But even those with the hammer mounted pins may give issues? That's where the experimentation comes in. I've got an old 4 screw K-38 that has a lock time you need a sun dial to measure, but it lights everything put in it. Some others to get that light, may only work with Federal primers? Grab some WinClean, S&B or MagTech among others if you want to try for real world adequate hit strength. So, what I do anymore is simply smooth the action aspects while keeping the standard mainspring in place, as others have already mentioned. I always try to avoid changes other than smoothing on a carry gun. Just for reliability.