How getting the angle relatively to a plan which passes through the object's center?

I want to translate an object along it’s symmetrical axis. I think of moving it defining the three world coordination axis considering the angle respecting to a virtual plane that passes through the object’s center. I have got some ideas; for example I would like calculating the sine and cosine of the angle, getting the local direction for the translation method. But to do that, I need the angle. Given that this script must work on 15-20 object spawned in the scene, I need something to get it with less calculation possible. So I’m asking if there is a method that returns it or a common way to get it with less calculations possible, because of the material to examine is so much and I have found nothing else the Vector3. Angle and Quaternion.Angle, that create me lots of problems. Unfortunately I’m a beginner and I don’t know unity a lot.

These objects are clones of a prefab that have got different and random coorinates.

 

Vector3.Reflect?

 

How?