Normalizing test data
Judging from your question you are probably using this formulae for normalisation:

(x - x_mean)/(x_max - x_min). This is just an approximation of the real normalization formulae. The real one would be:



where mu is the mean and sigma is standard deviation. If your data trends is same throughout, then you can expect the mean and standard deviation to be approximately same, and thus give you a more uniform representation.

Check this Wikipedia article, Feature scaling which says about the schemes used in different ML techniques. Hope this helps!