hi Marnie ~
The pen will leak if the needle is left screwed to the end of it. If the needle is removed after each use, the pen shouldn't leak. The rubber stopper in the pen is similar to that which is in a vial.
My concern is delivering a half unit of insulin. It seems to me that the pens with which I am familiar are graduated in whole units. I am looking at a syringe as I write this and it, too, is graduated in whole units. I don't know about a Novolog pen, but the half unit dose is something that maybe you would like to discuss with your endo or CDE.