The ROI calculator is based on the premise that when you use Email to Case Premium, you save 6 clicks per each outbound email response, at 5 seconds per click, which in turn saves an average of 30 seconds.
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The specific clicks that are saved include Select Template (2), Select From Address (2) and Edit Case Fields (2). And if you use Canned Comments, Knowledge or Attachments on Cases, your savings will be much greater.

Additionally, we assume that support agents work an 8 hour day and that there are 235 working days per year (255 business days per year minus 20 vacation days).