OHHH!!! so that is what that number is for... I thought it was spawn PROTECTION, not chunks...By default minecraft will always load x number of chunks around the worlds spawn point as a permanent chunk loader. Which last I checked can be configured in the server config file but for SSP has no config.
Zombe's modpack does a similar thing, except it shows yellow Xs where daylight means no spawn but nighttime means spawn. So you'd only have to watch out for red in a permanent daylight dimensionSomething to note though is it ignores daylight. Useful in the overworld since mobs could just spawn at night...but eternal day ages it shows as mobs being able to spawn most everywhere, even though it will never be night and actually allow it.