I use 1/2 cup corn sugar to keg. I agree, natural carbing gives better mouth feel; softer, smoother, that's why I still do it. I know weighing would be the more accurate way to go and maybe some day I'll learn how many ounces equal my 1/2 cup, but it's been working good for me. I don't really know the science behind it, but you should use less when keg'n vs bottle'n. I boil the sugar in 1 cup water and after racking I only add serving pressure (10psi for me) to seal the keg. It's probably even less psi because I don't leave it on very long. I then shake it a bit and check for leaks.
You'll see the color change as you tap, going from the yeasty slurry to your beer. After it sits for a while, it might happen again. I usually lose about 4 oz at the beginning of the first few sessions.