How to get the number of significant Eigen Value?

Im working in Matlab to compute the PCA. I already compute the Eigen Value and the Eigen Vector.

I used this matlab function :

   [Eigen Vector, Eigen Value]=eigs(Matrix,k);

With this eigs function, we will get ordered Eigen Value (from big to small).

In this case, Im going to determine the k (which is the number of Eigen Value I want to keep).

And I need to compute k with this formula :

    Σ(from 1 to p) of eigen value
   ------------------------------------------------------- * 100 = our persentage
   Σ(from 1 to col of eigen value matrix) of eigen value

where k=p

And to reduce the dimension of input matrix, I just have to :

       (Eigen Vector*Eigen Value*Eigen Vector')

Someone know if there is matlab function to get the k number based on the persentage I want ??