Centre of tribe restarts are calculated by adding all the x and all the y co-ordinates together then dividing them by the number of villages. The x and y result will be the approximate x and y co-ordinates of your village (if there is already something there, it will be moved to the closest blank square).
For example, if your tribe owned 0|0, 0|999, 999|0 and 999|999, you would have a village at 499.5|499.5 (i.e. either 499|500, 500|500, 499|499 or 500|499). Even though this village is the furthest away from your tribe's villages, it is the centre of your tribe.
The same thing will occur if your tribe has 100 villages in K99 and 100 villages in K9. You will end up somewhere in K49/59. The more villages there are in K9, the further north the village will go. The more villages there are in K99, the further south the village will go. (This rule can also be applied to East/West, NW/SE and NE/SW).