Steps per inch is a strict function of gearing and screw ratios. Once it is set up correctly, it is never altered for backlash.
Backlash is a certain amount added (or removed) from an axis movement during a direction reversal.
You can test as you were by drilling holes, drill one pair of holes in the normal method, then drill another pair on the same spacing, but insert an extra movement in front of the second hole, to cause the machine to first overtravel the second hole position, after which it will then approach the hole from the opposite direction.
Compare the seperations of the two sets of holes. The difference will be the backlash in that axis.
First you get good, then you get fast. Then grouchiness sets in.
(Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)