Cement Recommendations for Fence Posts
If you are going that deep in the ground, your fence posts will be plenty stable by placing and packing the dirt back around them. To pack the dirt I use a 2X4 with the end cut down to size and rounded for a better feel as a handle, and for tighter places I use a 1X4. I would only do concrete around them if you are setting a large gate to a post. 

If you must use portland, instead of the just add water type, the mix ratio is 1-2-3. 1 shovel portland, 2 shovels sand, 3 shovels gravel.