Reducing magnitude by x amount

This may be an easy one.

I've got a Vector, and i wish this Vector to be identical in angle, but 5 units shorter. Basically I;m looking to reduce the Magnitude by 5 units.

I've git working code, but I'm worried about using Normalize, and slowing things down unnecessary. Is there a better way of doing this?

 attackVector = attackVector.normalized * (5 - attackVector.magnitude); 

As always, thanks, you lot rock.