Unlike most houseplants, like aglonoema's, the variegated rubber trees prefer less light then the darker ones. That variagated one should not have any direct sunlight. Especially on spots that were already browned. I would move it further from window if its catching direct sun. The darker Burgundy rubber trees can take a couple hours of direct sunlight a day.
When watering I let it get pretty dry, at least couple inches down. then water till it drains out bottom. I use non softened filtered water that has sat out for 48 hours. I also prefer not to mist the rubber tree cause they easily develop water spots or guttation lines. Better to wipe the leaves down from time to time. Feather dusting is also good for these plants since they attract alot of dust easily.
