How can I find the "center" of a dungeon?

I recently found a dungeon in Minecraft, and I have decided to make an XP farm out of it. From watching youtubers do it, I know that you need to have an 8x8 hole around the spawner to maximize spawning space. Since this is an even number and there is only one (1x1) spawner, the spawner cannot be in the center. How do I know what side to put it on? I know that it matters what side you put it on, I just don't remember how I can find out what side.

Solution 1:

What I do is stand where the spawner is, and find which corner moving to causes both the X and Z coordinates to decrease. This is the corner that should be four blocks from the spawner. (Likewise, when both coordinates are increasing you should move three blocks out from the spawner.)