The Room is everything...
The room is probably the most important tool in mastering. We make our mix/mastering decisions based off of our room. How so?... Well when we listen to audio through our monitor setup we hear the audio reacting to the room. For example, a room may enhance the low end causing us to put less low end in the mix/master. We hear it as sounding good but in reality it is lacking or the reverse is happening. We can only make adjustments based off of what we hear. We want our rooms to be as flat as possible meaning the room has little effect on the audio coming out of the monitors. Listening is so critical in mastering that the room must not be neglected so that it does not alter the sound coming from the speakers.
Once the room is correct it gets easier, however we still have to "learn" the room because no matter the type of correction you have the room will always react to the audio but knowing how the room reacts will help you to make informed judgments in your mixes/masters providing you with something that translates well to other listening systems. The best way to learn your room and monitor setup is to listen to commercially mastered music in your studio. Most don't do this but get in the habit of playing music through your setup and just listen to it as your doing other stuff. You don't have to blast the audio but just enough to have a conversation over. Over time your brain will simply know how something " should" sound in your monitoring environment. It's the same reason that you know something doesn't sound right when you take your mixes/masters in your car. The car is the big one because most listen to music in their car more times than any other place. So if you listen in your monitoring environment more than any other place you will eventually know if you got it right or wrong.