You the user can enter your portals x, y and z location coordinates to calculate the location of the second portal.
The x, y & z coordinates can be changed to minus numbers with the use of the plus/minus buttons.
The users Portal locations can then be saved with the save button as a new location, and also renamed by the user to something like my base or my farm or anything the user can dream up and then be viewed sometime in the future by you the user.