I recently switched from gravel to an aquatic soil/sand mixture in my 90 gallon tank. I purchased a 40lb bag of soil and it covered about 2 inches or so in the front to about 3 inches in the back (could be slightly more).
While considering the sand, it seemed rather high but I figured the soil would be compacted even further once I added sand and water. Well, it didn't. My substrate is too high (for my tastes anyway). Incidentally, I added 75lb's of estes black sand to 35lb's of soil.
I don't know if flourite is the same composition as schultz's aquatic soil. If it is, then I would add maybe 20lb's of soil and 40-50lb's of sand. As SnakeIce mentioned, consider your plants and the depth. You may opt for additional flourite over sand.
BTW, I'll be switching back to sand. I was aware that the 2 mixtures would eventually mix together and it turns out that I don't like that look at all. Also, due to a combination of the types of fish I recently purchased and my darn severum's eating plants, I decided to plant less and to go back to sand.