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S11 is just the fraction of the energy that is reflected - this classic HP app note explains it very well:http://cp.literature.agilent.com/litweb/pdf/5952-1087.pdf
Another approachable text is Chris Bowick's RF Circuit Design, very approachable for engineers in practical terms, and thankfully free of unnecessary academic fluff.It seems to be available in PDF if you google it.
Your answer seems to be wrong. Off by a factor of 2.S11 is the complex reflection coefficient.Convert complex impedance to reflection coefficient, take the magnitude. Just Google it.Magnitude is 20*log(mag(S11)) because S11 is a voltage ratio.
Back in the days of QuickBasic I wrote a little program for this kind of thing and transferred it across to MSDOS back in the 1990s.You can see it in the black DOS window in the image below. The background is an old copy of Eagleware Genesys showing the analysis of your load impedance and it seems to agree with my ancient DOS program. It gives a result of -3.979dB for S11. You might also find some of the other analysis info in the image to be useful?
It's late and I'm tired and I don't have a microphone here but try this silent youtube vid that shows how to get the magnitude of the reflection coefficient and then the return loss for your chosen load impedance using a simple excel sheet.Hope it's OK. It will probably fail if you put zero in for the real part but you get the idea how it works. The opening few seconds show part of the initial start screen of my old DOS program.https://www.youtube.com/watch?v=eZmV4CeXobg&feature=youtu.beNote that I haven't done this vector based algebra since I wrote that program over 20 years ago so there are probably bugs in the way I've done it from memory. I always use a computer for this stuff. I do remember that I had to do some extra computation in the program to get the angles to display correctly for all possible load impedances.