Note: After clicking the slider once, you can use the arrow keys for more precise selection.
Item Grade always Exotic. Stone Grade always Exotic except Synthesis and stones are +12 for ring/belt. You can't choose same stone twice a time except Synthesis. You can only select the "Spirit Stone Reinforcement (Empower)" attribute just in the 4th stone 2nd attribute for ring/belt.
Powerstone calculator only calculates stones and calculates xxx increase %. It doesn't calculate the item base values (that's a bit complex) and the additional + powerstone value (I don't know how these values are calculated).
*Max Level Mod unlocks the option to increase the maximum stone level in sockets up to 33/36. This limit is 36 for weapons and armour and 33 for ring and belt.