Use something like this as the custom calculation script of "BMI Category":
var bmi = Number(this.getField("BMI").value);
if (bmi<=24.9) event.value = "Normal";
else if (bmi<=29.9) event.value = "Overweight";
else if (bmi<=34.9) event.value = "Obese";
else if (bmi<=39.9) event.value = "Severely obese";
else event.value = "Morbidly obese";