well, the question says that it needs to be within 10%... I am 99% sure that the answer is greater than 1600pm (the boundaries of the well) and 90% it's around 2000pm... I would try values between 1800 and 2100 with smaller increments maybe? if it's an online submission form that you're using...
Just helped a friend with a similar problem... so, I would agree with you that the left and right boundaries of the potential well appear to be around +/-400 pm. This is because of the change in the sign of the second derivative (which would be where the oscillating term, like sine or cosine...