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For a correct re-entry you should place your perigee so that it's just inside the high atmosphere. The higher your speed, the higher your perigee should be so that you have more time to "bleed off" speed in the high atmostphere. When it's time to deorbit, you have a very useful display in the flight computer to guide you in your descent. Press "D", then "2", and the de-orbit MFD display will show your predicted re-entry angle, range to ground, and speed. There is a tutorial in this doc that demonstrates a complete re-entry from the ISS using the autopilot and the correct instruments: Click here for the tutorial.