how to calculate mortar needed for flagstone

[dubb]

i was just wondering if any of the pro's on here have a method for calculating the amount of mortar needed that will fill the cracks for doing a flagstone veneer wall. what we will be doing is installing stone on some already poured concrete walls and columns. There will be a total of 30 tons of stone but what i need is to figure the amount of mortar needed to stick the stone to the wall and filling the cracks. we plan to keep all gaps at about 1/2 inch. any help would be greatly appreciated. thanks

[Stillwater]

mortar rule 38