I have done plenty of ferments in 5 gallon carboys. Don't worry. You won't lose anything to blowoff, except maybe a little foam. Marking your carboy levels is not OCD, it's smart. I do the same.
I recommend though that you shoot for a starting OG using a hydrometer, rather than using pounds of honey and gallons of water. Honey contains varying amounts of sugar and water depending on a variety of factors and so will require varying amounts to achieve repeatable results.
Pick a starting OG, say 1.120, start with about 3 or 3.5 gallons of water in the carboy, and add honey gradually, mixing thoroughly, and testing gravity until you get to your target OG. I find this is most easily done in a bottling bucket with a spigot, then you can simply gravity transfer to the carboy which aerates at the same time!
Hitting a repeatable OG, but missing your exact 5 gallon target volume is much better than having exactly 5 gallons of something you can't repeat if it turns out awesome.
