In the real world there is one number that one never wants to exceed and that is MTOW (MAXIMUM TAKE OFF WEIGHT).
Exceeding this number will result in making the headlines of your local newspaper under Aircraft crashes on takeoff.
something you want to get in the habit of doing is to all ways check your weight. A lot of freeware aircraft will have to much weight in them. I have yet to see an aircraft that does not have the maximum fuel in every single tank of the aircraft. This is hardly ever needed. I used to work at KJAX and fueled aircraft and the only aircraft that I ever filled to max was a MD88 going to KDFW (Dallas Fort Worth) and that was only because of the bad weather. And by doing that it limited the number of bags you could put on the aircraft and there was no room for any freight or mail at all. The plane left just below the MTOW. The center tank on just about any aircraft was hardly used but you have to know that most flights out of KJAX do not fly in real distance at all. Some times some of the CRJ aircraft would want some fuel in the center tank as a precaution. So unless your flights exceed 5 hours you will not need to have that much fuel on board. Most flight planners will tell you how much fuel is need for the flight you have selected.and you can add some extra fuel to be on the safe side most planes will do that.
One has to remember that anything you put in an aircraft will increase its weight. If a plane has to much fuel on board and for some reason or another has to return to the airport or land at another airport that was closer than the original destination they will have to dump fuel before they can land because they are too heavy. As you will be over that other number MLW Maximum Landing Weight.
These are the two numbers one must think off when flight planning