Another way to add glycerin to get an even distribution is to rack a 1/2 gallon or gallon jug about 3/4 of the way full. Add the all the glycerin for the batch (if doing 5-6 gallon carboy). Mix well with stir rod. You can hold it up to the light to see if you have all of the glycerin dissolved off the bottom. Then, siphon back into the main batch and stir. I usually do it under an argon blanket. This works really well. I always let small bench trial determine how much glycerin the wine wants (if adding). I try to keep it with the natural realm of what a high glycerol yeast will produce (9-12 g/L). I usually make an assumption my yeast produced 6-7 g/l.