I _believe_ it's at the upper edge of the curve when it transitions from vertical to diagonal. Therefore, the entire upper triangle would be considered extra room. I may be a little off on this but it's certainly not to the top of the carboy.
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Check out this video on YouTube:
I just measured mine and it is about 3/4" below the spot where it starts to curve inwards and narrow. Not sure if all carboys are the same or not though.
This is why I ask. Here's my mead I just transferred from my primary to my secondary. I think my primary was 7 gallons and I know my secondary is 5 gallons. So is this exactly 5 gallons? I dunno. I guess I'll find out when I bottle. 25-26 bottle of mead.....mmmm
I have a carboy that looks similar to yours. Just the other day I measured it so I would know where the 5 gallon mark was. It came out to be right around the highest band right around where it starts to taper.
I would guess you have a bit more then 5 gallons.