Which income box on my W2 do I enter as the "total income earned at this job" in the home office portion of my return?

You would use Box 1 of your Form W-2 to determine the amount of income earned for a home office.

If you worked for more than one employer, or had more than one W-2, and the home office was used for both, you would add together the Box 1's as reported on each form.