Questions and Confusions with headspace

I'm making a wheat beer and this time I put it into a glass carboy instead of the plastic bucket like I usually do. In the back of my mind I was remembering about how it is good not to have to much headspace. So I filled her pretty close (2 inches) from the top. As you can imagine, the next day beer was spouting out everywhere! Well, I got that taken care of, now it's about ready to go to secondary, but I see I'll need to add at least a half a gallon of water to limit the headspace in the secondary. Is that to much water? When is it time to worry about headspace? How do you solve the headspace problem?

Any help is greatly appreciated.

Remember it is about exposure to oxygen. During primary fermentation enough CO2 is produced to "blanket" your beer and protect it. Therefore, head space is not an issue. In fact, you want head space to avoid the volcano that you experienced. Some people even do primary fermenting in an open container. Look it up it is kind of cool.

For a "secondary" or conditioning there is not a lot of CO2 produced nor is there krausen. This in turn means you do not need and do not want much head room.

That being said many people may not even do a secondary for a wheat beer. don't add water, it would be too diluted. Keep it closed. Wait 2-3 weeks and bottle. See Revvy's thread about mistakes made.

+1 if you secondary it, don't top up. Just transfer it as it is. It'll give off some CO2 that will help protect it. Or just leave it where it is until bottling time and it'll be just fine.

I have 5 gallon carboys for secondary "fermentation". I put it in quotes because secondary is really conditioning the beer and not doing any fermentation. For my primary fermentations, I use a 6.5 gallon carboy. Having a lot of headspace for your primary is a good idea because you're going to have varying amounts of krausen. With all the CO2 that's coming out during fermentation, you also don't have to worry about oxidization. For higher gravity beers, I also have a blow off tube coming out of the carboy.