There is no ideal weight. It depends on numerous factors which those BMI charts don't take into account. Do you know what your bodyfat percentage is? That will give you a better estimate to an ideal.
You will most likely recieve many posts asking for your full stats and goals. Such information is needed before proper advice can be given. Are you a muscular 250lbs or more chubby 250lbs? That makes all the difference in the world. Since you are asking for help I will assume the last one. Not trying to offend, mind you.
What is your metabolism like? What is your diet like right now?
IF you can answer these, that would be a good start.
BTW, you don't have ANY access to weights?? In my opinion/experience you really need to. Adding weightliftiting to your program is a win-win situation.
Good luck,
Paulo