RIRO - Rubbish In Rubbish Out!!
If you put in all the data as per Airbus you will have a wonderful tactical and strategic tool, as the predictions will be accurate to destination and alternate which will enable you to make INFORMED decisions. Flying is about options - so you will be able to test options A to D and more via your FMGS, IF AND ONLY IF ALL THE DATA IS LOADED!!!
Fly safe.