Gravity dripper (IV drip)

I decided to try the IV drip method for fertilizing my plant tank, after
reading about it here. So I headed off to my favourite junk (surplus) store
to see what they had in medical supplies. I found a FLU VEN 0160 for $2.95
CND, which is rated at 20 drops per ml. I was hoping for 60 drops per ml,
but considered myself lucky to find anything.

I'm having problems getting a consistent flow rate though. My test setup
will drip OK for a while, but when I check it later, it has either stopped
or slowed considerably. My goal is about two drops per minute, which would
give me about a quart per week. Can this be accomplished reliably, or should
I opt for a higher flow rate and more dilution of my PMDD? I'm also not sure
if I need a vent hole in my drip container, but I'm guessing 'YES', since my
test container is collapsing as the liquid drips out.