will actually depend upon, amongst other things, the type of data that's being saved. Unfortunately you cannot guarantee that a 300Gb tape will be able to 'hold' 300Gb of data.
There have been a few other posts on this forum about the self same thing & I've usually quoted a few examples from our environment.
We use LTO3 tapes which have a 'native' capacity of 400Gb & 'compressed' capacity of 800Gb. Now, looking at all the FULL tapes in our library I can see that the amount of data held on the tapes varies from 360Gb to 1.2Tb - now that's both below and above the 'advertised limits' !!