How much honey to carbonate?

I'm going to do my first batch of mead soon, after I move the beer over to a secondary and make 5 gallons of soda (that'll take all of an hour to get to bottle).

I'm thinking I'll bottle 2.5 gallons of sweet mead, and then add extra honey and finish bottling to make about 2.5 gallons of carbonated sweet mead as well. I've never had either so I figured I'd make both, eh? Though I have no idea how much honey to add........

Think again. (Sorry That Sounds Rude) and I don't really want to be a party Pooper here.
There are only a few ways to get carbonated sweet mead.
1. Force Carb using a keg setup.
2. Use a yeast strain that will COMPLETELY ferment out the mead, backsweeten with NON FERMENTABLE SUGAR (Think Splenda) and then prime with honey or corn sugar.

other than that, it's not going to happen for you. The fact that the mead will be sweet is an indication that the yeast can not survive in the alcohol concentration in the mead...thus will be unable to consume your priming sugar and will not carbonate.

I think you are wanting to naturally carb some sweet mead. If its sweet, it is either not done fermenting or you have surpassed your yeasts ability to ferment. In the first option, it will keep fermenting until dry, not stay sweet. And the second option leaves no living yeast cells to carbonate.

Basically, you can not have residual sugars and bottle carbonate. The only way to naturally carbonate and have sweetness, is with nonfermentable sweeteners such as Lactose or splenda.

Well, you can bottle carbonate sweet mead, but it's called bottle bombs eventually :-o

I kind of expected that but figured I'd give it a shot.

This makes things easier though, no decisions to make.