In the most recent seasons of both Mindcrack and Hermitcraft UHC (season 16 for Mindcrack, and season 1 for Hermitcraft), they used one of the new features of Minecraft 1.8: a world border. This world border has a couple of aspects to it that make it dangerous. If the world border is shrinking (because it can do that), you can get caught outside the world border, and die, with a message saying that you suffocated within a block.
This post is about how to determine the position of a shrinking border, assuming that the center is at (0,0). All you have to do before the border shrinks is set the starting side length, the final side length, and the time it will take to get from the starting side length to the final side length. Then all you have to do after the border shrinks is give the amount of time that has elapsed since the border started shrinking. The formula will return the distance from (0,0) to where the border is.
NOTE: The elapsed time variable may be in seconds, minutes, hours, or anything else, but it has to be the same unit as the variable that marks the time the border will take to shrink. My recommendation is in minutes, but the Minecraft command for setting the world border is in seconds.
Here is the formula:
[i] = The side length of the border when it starts shrinking.
[f] = The side length of the border when it finishes shrinking.
[t] = The total time the border will take to shrink.
[x] = The elapsed time since the border started shrinking.
[y] = The distance the border is from 0,0
y = (0.5)*(i-((i-f)*(x/t)))
If you don’t want to have to apply the formula yourself, I have made a program with a built-in timer you can run in the background while you play UHC. Before UHC starts, you give the initial side length, the final side length, and the total time it will take for the world border to shrink. Then when UHC starts, click start. The timer will automatically start, and every second the program will get the amount of elapsed time and find the distance from the world border to (0,0). All you need to do is quickly switch to this program, read the distance, then switch back. You can download it here: World Border Locator.