2500 calories is an estimation based on what is generally considered approrpriate for an average female of average height and ideal goal weight. It isn't a fool proof bang on guarantee of actual requirements. That would be impossible. Everyone needs different amounts of calories depending on height, weight, age, gender, activity types and levels, emotional stress levels, specific food consumption patterns, macronutrient intakes in grams (eg. protein intakes), sleep patterns, sexual activity patterns, genetics, metabolic rates, medication use, etc etc etc. The best we can do is estimate.
That having been said 750 calories is very low for anyone. I assume you have selected that you want to lose 2lbs per week on cronometer in order to get that figure? The more calories your body needs the more you will be able to eat and lose weight. But if you eat them and don't need them, you will have a problem. Do some weight training daily and you will be able to eat a lot more.
I doubt that 750 calories could sustain anyone for long, even someone who is short with low activity levels and higher protein intakes. It would just be too hard to get enough nutrients.
I find the calorie guidelines in Dr Graham's 80/10/10 book to be sensible. This is because he does not espouse a "one size fits all" mentality which may not be optimal for those of us whose size is far outside of the norm.
i'd say nothing under 1800 or 2000
i'm 5'3 and 98 pounds and i never dip below 2500 because its not enough calories!
eat as much as you can:)
Cron-o-meter is a bit weird when it comes to that... I have the settings set on 80/10/10, you'd think the machine would understand that eating 1900 calories is way too little for me, but I'd say that 1900 is the BARE minimum that your body requires to remain alive... meaning that if you actually GET OUT OF BED, you need to up those cals haha, so since most of us also get into the shower, walk around and talk and do things we need to use our brains for (thinking requires lots of glucose :)), lots of additional cals are going to be added. Go by feeling, eat when you're hungry, sleep when you're tired.