Great info on the main site about pellets:
http://www.redearslider.com/pellets.htmlThe general guideline we give is the amount of pellets that would fill your turtle head each day for young ones and once they reach adulthood at around four inches in shell length that amount of pellets every other day.
Over feeding can lead to rapid shell growth and pyramiding if the shell--where the scutes aren't shed and form pyramids. In the extreme case, over feeding can lead to more serious health issues.
We got our turtle a little larger than yours, but not much. I quickly found this site and the advice I just gave you. Here's my more nuanced experience. Your turtle, at its size, will go through growth spurts. There will be times where feeding the amount that will fit in its head will be too much and times when it will be just right. Another bit of advice I found was to watch the skin between its front legs and the shell. If it's bulging out, you are over feeding. If it's too sunken in, you are under feeding.
And because of the uneven growth, you can't rely on a steady X amount of pellets a day.
But, use the what fits in the head/day as a guideline and adjust as you see how it affects your turtle.
At some point you will want to introduce vegetables. Check out the other nutrition pages on that link above. Right now your turtle doesn't need veggies, and may not want them. However, at some point it's growth will slow considerably. At that point you want to transition to fewer pellets and supplement with veggies. Before you reach that, you want your turtle to realize that vegetables are also food.
That transition should happen by the time the shell is 4" long. For me, it was between 3" and 3.5" because of slowed growth and the early signs of pyramiding since I was still feeding head/day in pellets.