To retain control and, most importantly, avoid injuries at an early stage of competition, it is generally advisable to stretch your racquet "fairly," between 23 and 27 kg. "Extreme" strain, defined as less than 23kg or more than 27kg, is typically difficult to handle and necessitates significant skill. The best way to do this is with a weight. Simply pull your racquet really hard, then let go of it. If it feels right, it's not too tight. If you don't have access to a weight, try standing over a chair and jumping up and down on it. That'll get the blood flowing and help you relax enough to take the pressure off your arms. Finally, if all else fails and your racquet is still too tight, you can always use stringing tools to loosen it up a bit.