I am an RN and I work in a Level 3 NICU. I totally understand your situation. I make ballpark times for my pumping break, and let my nurses around me know I need to go in that time frame. Everyone has been really nice about it with a few exceptions.. and those are rough days.
A few things that help me..
I pump in my car right before I head in for my shift. this way, I have pumped very recently.. and in case a baby codes or something.. I didn't totally skip a pumping, I just did a half pumping. It helps keep my supply up. I pump while i'm eating. I take a sandwich. So then I pump during my law required lunch break.
And then, I pump after I clock out, immediatly after, before I head home. If you got no breaks at all.. except for lunch.. you got your 3 pumpings in.
In addition to these pumpings, (I work 7 to 7) i pump at 9, 12, and 5ish.

So on days I work, I BF at 4am, pump at 6 am, pump 9-10ish, pump 12-1ish, 3-5ish, and then at 7ish after clocking out, BF at 8pm, BF 9 pm (May BF one extra time @10 to get baby to sleep)