Use issue_key (epic_key) to obtain epic progress.

HOW_TO.md

#To use a custom function:

• Create or open a spreadsheet in Google Sheets.
• Select the menu item Tools > Script editor. If you are presented with a welcome screen, click Blank Project on the left to start a new project.
• Delete any code in the script editor. For the PROGRESS_EPIC function below, simply copy and paste the code into the script editor.
• Select the menu item File > Save. Give the script project a name and click OK.
• All done! Now you can use the custom function.

PROGRESS_EPIC.gs

var epicProgressURL = "https://{host_name}.atlassian.net/rest/greenhopper/1.0/xboard/plan/backlog/epics?&rapidViewId={PROJECT_ID}";

var fetchArgs = {
  contentType: "application/json",
  headers: {"Authorization":"Basic [base64 of username:password"},
  muteHttpExceptions : true
};

function getPercentageCompleted(data, epicKey){
  for(var i = 0; i < data.epics.length; i++) {
    var epic = data.epics[i];
    if(epic.key.indexOf(epicKey) >= 0) {
      return epic.epicStats.percentageCompleted;
    }
  }
}

function PROGRESS_EPIC(input) {
  var httpResponse = UrlFetchApp.fetch(epicProgressURL, fetchArgs);
  if (httpResponse) {
      switch(httpResponse.getResponseCode()){
        case 200:     
          var data = JSON.parse(httpResponse.getContentText());
          // Check the data is valid and the Jira fields exist
          if(data && data.epics) {
            // Set some basic ticket data in your spreadsheet
            return Math.round(getPercentageCompleted(data, input));
          }
          else {
            // Something funky is up with the JSON response.
            return  "Failed to retrive ticket data!";
          }
        case 404:
          return "This ticket does not exist";
        default:
          // Jira returns all errors that occured in an array (if using the application/json mime type)
          //  var data = JSON.parse(httpResponse.getContentText());
          return "Error: ";
      }
    } else {
      return "Jira Error","Unable to make requests to Jira!";
    }
}