amino acids - How do you calculate or predict the charge of a protein at pH 7?

To calculate the charge at different pH:

At pH 3 K, R, H are + and D,E have no charge so add up all of the K,R,H in the sequence and that is your net charge at pH 3
At pH 6 K, R, H are + but now D,E are (-) so subtract one total from the other to figure if your net charge is + or -.
At pH 8 K and R are +, H has no charge and D,E are (-).
At pH 10 R is +, K, H have no charge and D,E are (-).

That can give you a general idea. You can estimate the charge at pHs in between.