100 lbs for the first 5'? And 5 lbs for each inch afterward? I doubt it. Might work if you are very light of bone with very little muscle mass.
And , this can vary due to your age. I is better to be slightly overweight than slightly underweight. This is recent research. The key word is "slightly"
I am much overweight and have gained a lot in the last two years. I can say that I dont' eat much and still gain weight, but when you really start looking at how little food "normal" people eat in a day, it can be quite the shock.
I think a super stringent food diary is a good idea, for a week. And try to eat as you nromally do so you can get an accurate picture.
Also, from one who has been overweight for the last 23 years, mostly, it WILL take it's toll on your body, even if you are active and strong now.