^^^^Not exactly the only reason bro. By putting ml of air per ml of test takin your keeping the vial at atmospheric pressure, not a negative pressure. By doing so yes it makes drawing easier. If the vial gets a negative pressure inside it's going to want to pull the plunger in eventually making it impossible to draw.
Let's see if I can make this question make sense....So if someone was to put two oils in one pin (as in my question before), then would switching which vial you draw from create a good enough pressure or would that eventually catch up with you?
I guess to clarify myself. Should air be pushed into the vial every time someone draws?
if it is 250mg/ml, then 250mg=1ml.
if it is say 300mg/ml, then 250mg=.83ml
if it is say 400mg/ml, then 250mg=.63ml
if it is say 100mg/ml, then 250mg=2.5ml
get the idea?
Dose you want divided by dosage on bottle = number of ml you should draw.