You may want to consider getting a Digital Hygro Thermometer...it can retail as cheap as S$28 to $3+ depending where you get it...you'll probably get it cheapest from Giant; you can also purchase one from one of those well-stocked D.I.Y. shops!
You don't calibrate your dry cabinet; you set the desired level. You can calibrate your built-in hygrometer, that is if it comes with the calibration function. Most built-in hygrometers do not come with that as they don't come cheap. But there is still a way to calibrate it though!