Ideally, all should aim to keep their waist measurement less than half that of their height, found the scientists. That means a 6ft (72 inch) tall man should aim to keep his waist less than 36 inches, while a 5ft 4in (64 inch) woman should keep hers under 32 inches.They have found that the easy-to-calculate ratio between the two is a better predictor of risk than the most widely measure of obesity, called body mass index (BMI).hat means a 6ft (72 inch) tall man should aim to keep his waist less than 36 inches, while a 5ft 4in (64 inch) woman should keep hers under 32 inches. They have found that the easy-to-calculate ratio between the two is a better predictor of risk than the most widely measure of obesity, called body mass index (BMI).