The rockier and firmer the ground where you are putting the stone the less gravel base you'll need. Here in the NW I use 4-6 inches of gravel and 1-2 inches of sand. The sand is just a leveling material,so figure just 1 inch. The more you compact the gravel the better off you'll be. Use a crushed gravel that is 7/8ths-1/2" down to fines(matereial the size of sand), not a round gravel such as pea gravel.
Good luck

I have decided to use 2" of gravel and 2" of sand and then the 1.75" flagstone. My next question is how deep am I supposed to dig? Would it be 5.75" or 4" and move the sand around the flagstone? Any help or tips are appreciated.

I would dig out 4inches and use the sand as your varying material, because unless you are putting down brick pavers the thickness of the stone is always inconsistant.

Thank you for your suggestions. I was thinking to dig 5 inches. It will save me some work. Also, I would like to have ground cover,like scotch or irish moss between the flagstones. Any recommendations on what type of plants and if they grow in sand or must I put soil in it.

Thanks again.