What is the correct way to calculate survey completion rates?

hallaodeh ,
Here is my two cents. While it seems that calculating response rate and completion rate should just be a simple mathematical formula, it is notoriously difficult for any technology platform that I've had experience with to calculate. With Qualtrics specifically, the "funnel" technology is great for surveys with less than 1,000 responses, but with larger respondent samples it can really slow down the performance of the dashboard, almost to the point of unuseability. Your screen shot above shows a very simple approach to "completion rate" and I'd offer that you are on the right track. Keeping this as simple as possible is a prudent approach. Multiple your sum by 100 to get the percentage.

hallaodeh,
To answer your original question, using started would be correct, as you have outlined, not finished + started. We actually use count vs sum:
Of note, it doesn't look like you have any filters set on the calculation. Each field (completed and started) have a range of 0-1 with 0="false" and 1="true". You'd need to set your filters to "1" for each so that you're counting completed (True) over started (True).