I have been trying to adjust the belt tensions because I have an ovality problem with circles but I have never been successful. When I adjust the belt tension by measuring 150mm of the print head and using the printables https://www.printables.com/model/1017034-k1max-belt-tension-dial-indicator, there is about 3-4mm of play on the left side when I bring the gantry all the way forward. but if I loosen the belts completely, there is no play at all. What is the proper way to adjust the belts?
There is no proper way. You have to figure it out yourself and bring tension left and right as close as possible at about 110 Hz.
Use the screw at the spring tensioners. Somewhere on internet you can find a video how to tension your belts.