Thank you for your suggestion!
You see, "Estimated time" is a tool for planning and "Actual time" is a tool for tracking, that is why they can't be combined.
"Estimated time" function is for planning a possible time period you will spend on the particulat task (for example: 1 hour)
"Actual time" function is for counting the period of time you spent on the particular task in general (it can be as 1 hour, so as 4 hours).
So, if you plan that you will spend 1 hour on the task, but you spent 4 hours on it, the percent of "completion" will be 400.