@Chee Lon "My objective is to do an estimation on how much storage space I could free up after I have deleted X amount of records"

Again, you need to clarify - free for what? Deleting rows will not reduce space allocated to table and therefore tablespace free space will not increase. We will just end up with more unused space with table segment (subject to Bruno's explanation), so if your goal make more free space for other objects in the tablespace - delete will not do. If you want to delete some rows, compact table data and return freed space back to the tablespace you could use ALTER TABLE MOVE. You will need to come up with keep rather than delete condition. For example, you want to delete rows

where Date_of_Birth < DATE '2000-01-01'

This means we want to keep rows

 where Date_of_Birth >= DATE '2000-01-01'

or, if Date_of_Birth can be NULL

 where Date_of_Birth >= DATE '2000-01-01' or Date_of_Birth is NULL

Now we can:

alter table employee
  move
  including where Date_of_Birth >= DATE '2000-01-01' or Date_of_Birth is NULL
  online
  update indexes;

Still, even online move can affect performance so you'd need to test it in PTE (performance testing environment) before implementing. Another option would be deleting rows, enabling row movement and using shrink space but I don't recall if it supports online option.

SY.