Well, there's only one accurate answer you're going to get, but I don't think it's going to help you. You're overpitching when, due to the high levels of yeast pitched, you're getting worse beer than if you had pitched less.
Since "worse" here is dependent upon your expectations and later evaluation of the finished product, only you can answer that question. It's also going to depend a lot upon what yeast you use, what the recipe is, how much you aerate, and so forth for many factors. See? Not very helpful, but very accurate.
As for practical advice, here are a few thoughts. You're not pitching that much above "standard rates," so I don't think there's any immediate cause for concern. Do you detect anything off about your beers? If not, carry on. Your increased rates will definitely be changing the beer, but maybe that change isn't bad, or maybe it's just not significant to the flavor/experience of the finished product. If you do detect flaws, or you just want to test that variable to see if you can improve things a bit, try pitching 1/3 of the starter instead of 1/2 next time, or measure a bit more accurately and do a split batch. Experiment and see what happens. If that all sounds like too much trouble, then like I said earlier, in so many words, if it ain't broke...
*Member: The HBT Sweaty Fat Guys Cigar Club