Consider using the Tracking table. It allows you to enter actual start (Feb 1). Actual and remaining duration - which will calculate % complete.

If the remaining portion of the task will not be completed as scheduled (following on with Feb 2) then you need to split the task and reschedule the remaining duration to a more realistic time frame.

Another suggestion to avoid the split task - is to create the beginning part of the work as "preparation" and the remaining 20 days as another task.