You are correct this form is for one employer and the income from that employer only. The line 1 amount for worksheet C is only the wages from that single employer. The idea is to produce one form that accounts for all allowances; this is usually for the employer providing the largest income. The other forms for the other job or spouse's job will have zero allowances and thus max withholding for those incomes.
Line 13 will therefore only be the estimated withholding for that employer.
Additionally, for line 8 on the worksheet you would be using the table titled "Single Persons, Dual Income Married or Married with Multiple Employers."
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