I don't know the exact number, but three or four inches forward will put you in the ballpark.
I had an older boat with the boom bale in the original position. I wanted to install a second bale farther forward, but figured I'd test the location out before drilling the boom. I took some modern high-strength cordage (1000 lb breaking strength or so), tied a few loops around the boom and then tied the loops to the existing bale to keep them from sliding forward. I figured that way I could re-position the test bale until I figured out where I wanted it. In the end, it worked so well that I never touched it. Only took it off when I sold the boat because I figured someone would look at it and think something was wrong with the boom.