in the Star List: find the X, Y and Z for both stars.
find the differences between system 1 and 2 for each of X, Y, and Z
Square the differences.
Sum the squares.
Square root the sum of squares.
D= ((Xa-Xb)2 + (Ya-Yb)2 + (Za-Zb)2)0.5
where
D = distance in LY
Xa = system A's X coordinate
Xb = system B's X coordinate
Ya = system A's Y coordinate
Yb = system B's Y coordinate
Za = system A's Z coordinate
Zb = system B's Z coordinate
I'm too lazy to look up page references right now.