Where it gets very difficult to predict with displacements is that the junior pilot remaining in a BES that has displacements on the posting is not necessarily the junior pilot at the end. If 1 senior pilot volunteers to be displaced the senior most pilot that was displaced now becomes the junior pilot in the base. This dynamic number is what determines where you can bump to. So, if it looks like you can't hold a BES because they have bumped all the pilots currently in that BES junior to you doesn't mean you should not submit that BES as your preference. When they run the displacement it is what you can hold when they get to your seniority number, so if pilots senior to you have volunteered, you may not be displaced at all or may be able to hold a different BES that it didn't appear you would.

Here is my simplified example:

BES A
sn 10000
sn 10010
sn 10020
sn 10030 displaced
sn 10040 displaced

Let's say you are in BES B and your seniority number is 10025 and you are being displaced. It looks like you can't bump to BES A. However, if the pilot that is sn 10020 decided to volunteer for the displacement, then the pilot sn 10030 becomes the junior pilot in BES A, so you actually can.

As you can imagine when you have a large multi base displacement, the bottom numbers can change a lot as some people would rather move to a new plane than remain really junior in their current position.