How do I figure out how much of a calorie reduction it takes for my body to go into "starvation" mode?

We've heard it a million times - starving yourself doesn't work for fat reduction. Depriving your body of too many calories leads the metabolic rate to decrease (the body's defense mechanism against starvation) and when food is reintroduced, most of the food is stored as fat. But is there a test to know how much one would have to reduce in order for the body to slow down? Of course, you'd want to reduce caloric intake below caloric expenditure to loss weight, but how much? Is there a formula? I've never seen this question answered. Thanks for helping!