compute a difference( dB) between two BER curves
For the segment you show, there's no single number that describes what you want – your lines need to be parallel for that to become a single number.
So, your question can't be answered in general from the diagram, as you did, you'll have to take a fixed BER; and then, I don't really understand the question – that's a horizontal line in your plot. You can read the difference directly from where the curves hits that line.
You can, however, talk about an asymptotic gain if you are able to figure out how these curves continue for large SNRs (you'll have to run with sufficiently many points at say around 23 dB Eb/N0, in my experience, for 16-PSK); at some point, these curves become sufficiently parrallel. Then you can measure the difference in Eb/N0 necessary to achieve the same BER. (that's a horizontal line through the graph; you measure the distance of intersection of curves with the horizontal line.)
Note that this does not work the same way for BER achieved with a given Eb/N0. (i.e. you can't just measure the vertical distance, that doesn't converge as well.)