Springarm offset in length?

I’m trying to achieve that the camera’s springarm does not collide with the object (tree in the image), but when the camera is inside or to close to it, it’s supposed to be between the object and the character. My blueprint already works quite right. The problem is that the unfixed camera position obviously changes because my target arm length does. Is there a way to set an offset to the arm length in order to keep the ‘original’ target arm length?