What Weight Should Your Chelonian Be?

I would like to know where to find a weight chart or weight range for
aquatic turtles. I have a Mississippi Map Turtle who is doing very
well, but looking a little too well fed. I weigh him on my gram scale
every time I clean his tank. He ranges from 168-170 grams and he is 4
1/2 inches long. He is 5 years old and stopped growing a year or so
ago. I'm concerned about keeping him at an ideal weight range for the
next several decades! :)