Detecting incorrect disc size

hiya all, I am using Nero to burn dvds using dvd+r discs but nero is only showing the disc has a capacity of 4.3 gig when in fact it has 4.7 gig capacity? Why is this? This 400 megs or so of missing capacity is causing me to split dvds into 2 dics when i should be able to 4.5 gig movie onto 1 disc? wrote to Nero about this 5 days ago, but no answer. Using win xp pro, compaq presario 5423us, 1.6 g p4, hp dvd writer 200i with firmware updated to the newest one. really need this missing data space. thanx. Lou