If you can adjust easily enough, I'd say begin with 13mm & work your way down if needed. Otherwise, stick with the 12.7 that you're used to. IMO, shaft diameter is crucial because of how the shaft fits your bridge. I believe many things are affected by having a shaft too large or too small, such as stroke accuracy and deflection. Of course anything can be argued, but I'm not going to engage an argument. These are my beliefs based on years of building & handling thousands of shafts, and nobody will convince me otherwise. If the shaft is too thick, it's grabby in your bridge, as well as making it difficult to hit where you think you're hitting. If too thin, it's loose and sloppy in your bridge, as well as overshooting your intended contact point on the CB. A thick shaft that is tight in your bridge can assume the mass of your hand, significantly increasing CB deflection. A thinner shaft with wiggle room will deflect away from the ball rather than forcing the ball to deflect. There are variables to consider such as stroke length and the length of shaft protruding from your bridge at point of contact with the CB. This is all stuff that doesn't get factored into or considered with LD shafts. LD shafts reduce tip end mass, but do nothing to address other variables. Again, this is my opinion, could easily be argued and/or seen differently, but my experience tells me this is the way it is. Take it for what it's worth.