Hi Waldo Niemand,
You should create a Entity A to Save Percentage value and days.
Then create Another entity B for Parcel Entity and create 1-* association between A and B.
A is the master entity where admin will only Create/Update Values.
Now Add a Reference Selector in the Page.
Add a OnChange Micorflow for the reference Selector and write your Logic there to AutoCalculate the Duration.
Hope it Helps!!!