How does Milestone completion percentage works?

Background:

I have a milestone with the following data:

image

  • 35 total issues
  • 25 open issues (10 closed)
  • Displayed completion percentage: 39%

However, If I have 35 total issues and 25 remaining I have completed 31.25% and not 39%.

What questions are you trying to answer?

  • How is the 39% calculated?
  • Where can I check for full documentation? I checked here and I could found no information on this. Can you point me to a more detailed dcumentation?