Worldborder coordinates into scoreboard objective

The solution is quite similar to this answer I gave a while ago, but of course with some slight changes.

First things first, you need a 20Hz fill clock. How to make one isn't exactly within the scope of this question.

Next, you'll need a scoreboard objective to hold the current world border size:

/scoreboard objectives add BorderQuery dummy

You'll also need an empty player that will keep track of day time:

/scoreboard players set #BorderPlayer BorderQuery 0

The first command block to be triggered by the fill clock should query the border size from the game:

worldborder get

We need to redirect the result of command block to the scoreboard objective we set up earlier. Turn off the fill clock, stand directly on the command block, and enter this command:

/stats block ~ ~-1 ~ set QueryResult #BorderPlayer BorderQuery

The fake #BorderPlayer now stores the world border size in his BorderQuery objective. From there, you can test the objective, and with some more work, perform an operation on it.

If you need to do operations with the objective, it may be better to set the objective for @a (instead of just #BorderPlayer) in the stats command. Additionally, everyone will need the objective active.