User input vs calculated formula

Forgive me, I am not the most experienced with spreadsheets. I have always figured out how to make them work in the past, but I am trying to do something that may not even be possible. I searched for it in Google and on this site, but have not turned up anything.

I am trying to make several cells that can either be filled in by the user, or by changing a dropdown box in another cell, can be filled in by a formula. I would like to make this seamless for the user (i.e. not have the user have to input info elsewhere), if possible. The dropdown box basically says calculate or input. I am not too worried about keeping the dropdown box, if there is another way to make it work that accomplishes the goal.

I am using LibreOffice, English version, and trying to do this in Calc. I have Windows 8.1, English as well. If necessary, I could upload a sheet that shows what I am trying to do.

Thank you so much in advance for all of your help!