Your question is difficult to answer as what is 'BEST' for one person could be terrifying for another. I set most of my stuff up to be very twitchy so like a rear ward CG.
Mind you a 1/3 Spacewalker is not likely to get very twitchy.
Why not start at 4.5in and GRADUALLY move it back till you get uncomfortable with the elevator response and then go back a step.