[cxwrench]

Best way to do it is get one of them in a position that you like. Generally, the tip of the brake lever is about even w/ the bottom of the drop. This is basically what @mikerp posted, but there is no need to remove the bars from the bike if they're already mounted. You can hold a straight edge against the bottom of the drop and get close that way. Fine tune the position of that lever til you like it.
Here comes the good part...
1) Run a tape measure from the bottom of the lever body around the drop to the end of the bar.
2) Make the other lever the same distance from the end of the bar.