Question about IV pumps

We have an old one and a new one that we use for our abx infusions, which go through the venous chamber. Being a new nurse, I have a couple questions (i have seen it done 2 ways!)
The bag is 150ml NS, let's say we add 10ml for reconstituted abx. This equals 160ml, which I dial into the machine for infusion over an hour or whatever. If I put in 160ml, then at the end, there is still some left in the tubing. Pt does not get all his abx.
Same scenario, instead we dial in let's say 180ml, to help push some more through the lines to "get it all in". Eventually it is going to be sucking air through the line from the bag, which is going to trip the air alarm on the iv pump.

What is the correct way to do this so the patient get all their meds? Is the fluid left in the line considered an "insensible loss?" Surely not.

thanks for your thoughts!