What proportion of a padded FFT should be actual values
Be careful with this in thinking that you would increase your frequency resolution- you won't! Zero padding is very effective in iterpolating more samples between the samples you have, but it does not add any more information about the frequency content of your signal between your original bins.  You do not lose anything either.

The frequency resolution in all cases is $1/T$ where T is the time length of your data. 

Please see my answer at this post specifically for more details on zero-padding and how it works: What happens when N increases in N-point DFT