Commercial flat roofer here with 18 years experience.
I cannot stress enough how useless and incorrect inverted roofs are.
Having the XPS panels ABOVE the roofing membrane continues to be one of the dumbest things that I see on specs.
No matter what the insulation manufacturer tells you, those insulation panels will eventually absorb water. I have removed hundreds of these systems over the years and after 7-8 years of the insulation constantly sitting in water, they begin to absorb it.
And sit in water is what they do, they cover the roof and never allow it to dry.
I have removed inverted roofs with 3" XPS and those 2'x4' panels can weigh up to 20 lbs once they suck up enough water.
Once this happens, all the R-value that they once had is completely negated.
Putting the insulation over the roofing membrane is like putting your car seat on the roof of your car to drive and wondering why your cold and wet.
In most residential situations it is usually better to install 7" ROXUL on the inside between the joists. The only time I see different is if the owner is looking for an exposed ceiling look.
If that is your case, I would suggest a Ballasted 60mil EPDM system over two staggered layers of 2.6" polyisocyanurate (Poly iso), this will give you an R-Value of 30.
If you are determined to use and inverted system, I would highly recommend installing a dimple or drainage board over the roofing membrane and ensuring you are using XPS panels that have pre-cut reglets in the bottom of them that will help with drainage and help keep the XPS out of the water.
If you are looking for an r-30 you will need about 6" of XPS.