I think you have to take a look at all the variables, learn what part each of them plays, and go forward from there.
The first thing to look at is how are you going to apply the glaze. The conditions are different if you are going to brush the glaze rather that spray or dip it. I brush all my glazes and so the specifics here are going to be about brushing. But the generalities apply to spraying and dipping too.
Next you have the glaze itself and most crystalline formulas have very little clay which means that they are not really going to act very well no matter what, if you don't do something. But you can do something and the glaze will be fantastic to use.
I have been working with a material called Bentone EW and it looks like it is going to be the only clay that I will need in my glazes, and it also looks like the glaze will be really nice with only .5% Bentone EW. Imagine a glaze with .5% clay which stays in suspension and brushes well.
According to my friend Andreas Widhalm, Bentone EW is 10 times as powerful as regular bentonite. Bentone EW is what will suspend the ingredients in the water. It's much more powerful than Veegum T or Bentonite or Macaloid.
I mix up glazes in increments of about 300 grams dry materials. To this mixture I added 1.5 grams (.5%) of Bentone EW, dry mixed with a fork, and then added 240 ml of purified water and this made a very brushable glaze with the addition of some CMC solution. I think I can actually take out some of the water and have an even better glaze. Next time I will try 200 ml of water. I was able to brush the glaze on much faster than ever before.
One of the major points is that the more water you have in the glaze, the longer it will take to brush it onto the pot at the appropriate thickness. So, if you are smart, you want as little water as possible so long as the glaze brushes wonderfully.
Adding CMC will help the glaze to brush well. If you feel like your brush is dragging on the pot, you either need more CMC or more water. Again, according to Andreas, if you want CMC to thicken a glaze use it at a molecular weight of 300 or greater --- CMC with a molecular weight of less than 300 will thin a glaze. You really need CMC for brushability.
Finally for each different glaze you will need a measured amount of water, added CMC -- I make mine as a solution, and Bentone EW.
As an upper limit on the Bentone EW, I have a glaze with 2% EPK and 1.5% Bentone EW. I have added about 500 ml of water to get it to brushing consistency -- it also has a lot of CMC -- and I can brush and brush and nothing happens much except that the pot gets wet. Almost no glaze material shows up on the pot. I applied about 20 coats this afternoon and only got about .005" of glaze and that is not enough. I will have to throw that glaze away as it is useless. Its consistency is actually thick so don't let thick or thin fool you in how much water is in the glaze.
And I guess that's why I feel like specific gravity is not the right way to look at this problem. Specific gravity actually is used as a measure of solutions and a glaze is not a solution, it's powder hopefully suspended in water, which, I guess, makes it a compound.
I hope this helps. I would encourage you to keep good notes on how much water you have in each glaze batch, and how well that batch worked for you in application.