Here is a response from a friend now I just need to ask him to decipher it

The amount of energy the plate has to absorb depends only on the initial and final speeds of the bullet. If the bullet completely stops, then the plate has to absorb 1/2 * mass * speed ^2 (speed squared, or speed * speed).

Now, if the bullet comes in at a glancing blow, at some angle other than 90, then the bullet will leave with some speed, so the amount of energy the plate absorbs is smaller.

The question is, what is the final speed of the bullet? That's not a simple question to answer. One assumption, or approximation, is that the bullet loses all the speed perpendicular to the plate, but keeps all the speed parallel to the plate. In that case, the speed after the impact is v*cos(angle), where v is the initial speed. So, the amount of energy the plate must absorb is given by:

1/2 * mass * initial speed^2 *( 1 - cos^2(angle) )

When the angle is 90 degrees, the amount of energy absorbed is all the energy in the bullet. When the angle is 45 degrees, the amount of energy absorbed is half the initial energy of the bullet.

This assumes that the plate absorbs all the momentum perpendicular to the plate, but the bullet keeps all the momentum parallel to the plate. I don't know if that's a good assumption or not.

You can plot 1 - cos^2(theta) to see what the curve looks like as a function of the angle theta.