If your RDI for losing weight is 2500, then start eating 2500 and see how it works for you. RDI is very specific for each person's weight, so comparing with other people won't help too much. Generally, though, folks who are eating 600-700 calories are taking a lot of risks, and the body gets confused by intake that is too low. I wouldn't worry too much about what they are doing.
So start with Fat Secret's recommendation for you. If, after a week or so, you are not losing weight, you can try some other RDI calculators. I like these two:http://www.webmd.com/diet/calc-bmi-plushttp://www.fat2fitradio.com/tools/bmr/
They might give you slightly lower numbers, especially if you increase your loss from 1 pound per week to 2 pounds per week. (Fat Secret does not allow this option.) If you need to work with a lower number, you can set it manually in FatSecret and still use all the tools here.