if you are shooting 60fps then the slowest you can make it is %40 slower then original speed. so if you are using cinema tools and conforming then conform it to 24 fps.
what is easier to do though is in final cut (i assume that is what you are using) just change the speed to %40 and uncheck the frame blending. i like this option better because you have an choice to go back to original speed.
RIP Sarah Burke, CRJ and Shane McConkey