I have to jump in and ask as well. I have a pouch of glycerine and it states to add the contents to a 23 liter carboy of wine. The pouch is 30 ml, which is about 45 grams or 1.5 oz. and this seems small compared to what you are suggesting to use. So 4-5 oz or 5-10 oz is the norm for adding to 6 US gal?
I have not used any yet, but am making a sort of a port and was going to add some but now I'm unsure how much to add.