What Is an Easy Way to Quickly Estimate Water Loss

What is an easy way to quickly estimate water loss and fluid needs following physical activity? 

A)  Multiply body weight (kg) by 1 percent to determine liters of water loss 
B)  Multiply duration of activity (min) by body weight (kg) to determine mL of water loss 
C)  Subtract air temperature (°F) from body weight (lbs) and then multiply by 5 to determine mL of water loss 
D)  Take the difference in body weight (lbs) before and after the event and multiply by 2 to determine cups of water loss 
E)  Take the difference in body weight (lbs) before and after the event and divide by the relative humidity.