Anodized aluminum cannot be marked. It would require many thousands of volts to break through a good anodized surface. The alternative is to etch the aluminum before anodizing. If the etch is deep enough it will show through with a light anodized coating (not black). Since aluminum is a good conductor of electricity, it etches deeply very quickly and easily.
There is no formula with which to calculate the proper depth of the etch to insure it will last through the anodizing. This is because the various aluminum alloys are anodized differently.
There is, however, a simple way to determine the depth of etch necessary to show up after anodizing Start out by etching a piece of aluminum for two seconds time dwell, and five seconds time dwell, and ten seconds time dwell. After processing the piece of aluminum through the anodizing, one can tell which is the proper etch time.