Park Tool FR-5.2 fits Shimano CS-HG50 8 speed road cassette? Am I correct in saying that the Park Tool FR-5.2 cassette lockring tool is the correct tool to use on a Shimano CS-HG50 8 speed road cassette?
Yes, that tool fits all Shimano and SRAM cassette lockrings (as does the FR-5.2G and FR-5.2GT variants). 

https://www.parktool.com/product/cassette-lockring-tool-fr-5-2?category=Cassette%20%26%20Freewheel


  Fits Shimano®, SRAM® (including 1x), SunRace®, SunTour®, Chris King® and other cassette lockrings