In Chris Berry's "Two hand tapping of Scales and Modes (part1)" He suggests using finger 123 of each hand to tap out a scale or four notes per string. When using more than one finger for tapping scales such as in the tutorial mentioned, where should the right (tapping) arm be placed to make for easiest/fastest tapping.

http://www.guitartricks.com/2000/trick.php?trick_id=3079