"The center of a village is the centroid / average of all the doors coordinates"
(https://minecraft.gamepedia.com/Tutorials/Village_mechanics#Terminology)
What I'm wondering is which point is this referring to? The block below the door, the lower half of the door, or the upper half of the door?
If anyone is wondering, I found out (kind of):
https://minecraft.gamepedia.com/...m#Spawning
"centered vertically and horizontally between the bases of the 21 or more valid doors in a village if it has at least 10 villagers."
This, combined with the fact that the golems spawn near the centroid, means that it's definitely not the top of the door. The difference between the base block and the bottom of the door is negligible when making iron farms.
If you really want to know, I don't really have an answer for you. My best guess, based on the wording, is that the centroid is calculated using the bottom half of the door, not the base bloc.