I am trying to make a holming projectile using heartbeat event, so im describing its positions and orientations with CFrame. Its working well but now I want to add holming effect, and I have no idea of how to make the Orientation add half, or 1/5 of the way to facing another part direction.

I know how to describe the orientation to face another part in a single row in plenty ways, but half? Help

This is a perfect scenario to use CFrame:Lerp since you are already using CFrames in a heartbeat event.

CFrameCFrame:Lerp ( CFrame goal, number alpha )
Returns a CFrame interpolated between this CFrame and the goal by the fraction alpha

In your case for adding half a rotation, you would set the number alpha to 1/2 and you will obtain half the rotation of the projectile.

local projectileCurrentCFrameRotation =projectile.CFrame -projectile.CFrame.Position
local addProjectileRotationCFrame = projectileCurrentCFrameRotation:Lerp(goalRotation,1/2)