Let the Blend Tool Do the Math

Like a built-in calculator, the Blend tool (W) can do a form of calculation for you. For example, you draw two lines some distance apart, and now you want five more lines evenly spaced between the two original lines. Rather than doing any math to figure out how far apart to space the lines, just create a blend between the two objects. With the Blend tool, click on one line, avoiding anchor points. Then, Option-click (PC: Alt-click) on the other line. In the dialog, enter the number of new lines you want for the specified steps. Illustrator does the math and evenly spaces the new lines between the two originals. (And, since the effect is live, the spacing will automatically update if you move either of the two original objects, or change the angle of one of the lines.)