Starvation alone (assuming not combined with dehydration) is not solely based on weight and BMI. Starting at a lower BMI gives you less of a threshold as far as stored energy sources, but that is not the only factor in starvation induced death. Starvation affects the electrolytes and fluids in your body. You are no longer receiving adequate vitamins, electrolytes, and other nutrients. Your potassium, calcium, and sodium are just three factors that may cause health problems in starvation. How quickly starvation impacts you depends on what your nutrition status was prior to this, the degree of deficit you are in (ie eating 800 calories a day vs 0 calories a day), your activity level, any underlying health conditions, and to a certain degree yes your weight. It is impossible to calculate based on your height and weight alone.