Nero overburn help needed


 i have some overburn questions here, i m using nero and my samsung writer supports overburn as shown in the nero recorder info ...

 and i turn on overburn at the expert preference tab

 i bought some cd-r which says it is 790 mb and 90 mins, but the medium info says only 705 mb and 79 mins .... is this normal ?

or this cd-r somehow dun work with my writer for overburn ?

 one more question, after i click on the burn button to write a cd, if the file is too big, nero will prompt to finalize after buring or not ... whether i click yes or no ... it will start burning ... how do i cancel before it start to burn ?