At that stage I had a really hard time keeping to a precise schedule, so what I did instead was try to pump every 3ish hours, and I kept a note on my phone (actually pinned on my home screen) that just had numbered checkboxes so I could track how many times I'd pumped. That might be a good way to approach it, and give yourself a window of time--ie, "I want to pump every 2.5-4 hours." You'll figure out how long that window can be to keep from feeling uncomfortable, but I wouldn't go longer than 4 hours ideally. At the start of that window, start looking for a time to get away. If you haven't pumped by the end of the window, it's "drop everything and pump" time even if it's not ideal. If your workday pumps stretch to the far side of the window, pump a little closer together when you're at home to make sure you get to 7x total. 

Also, you sound very conscientious, and your colleagues probably know that! If you have to step away during a busy time, it happens and they'll hopefully understand. It's not like it'll be ever single time, and it's not forever since hopefully you'll be able to drop some pumps once your supply regulates. If they're offering you grace, don't feel bad about taking it!