Project was designed to calculate a variable for you, but I think you can do this:

1. Go to Tracking Gantt View
2. Go to the Tracking table (if you have trouble getting to the correct view AND the correct table, let me know)
3. in the table, right-click on Act. Start and choose Insert field... choose Baseline Start
4. right-click on Act. Start again and this time choose Baseline Finish
5. Right-click on Act. Start again, this time add Baseline Work
6. Move columns so that the first 7 are in this order: Baseline Start, Baseline Finish, Baseline Work, Act. Start, Act. Finish, Act. Work, % Comp.

Now think of the baseline fields as what you are calling "estimates"; (baseline work is "est_hrs").
Fill those in and then when the task is complete, enter the actual start, actual finish, actual work, and lastly, type in 100 in the % Comp field.