You can put an elastic band around the waist to mark the correct placement. The clinical limitations of bmi should be considered.

Understanding Your Qardiobase Measurements Qardio
Factors to consider when taking body measurement. Emotions sit at the top of the list because everything we do is driven by how we feel especially when it comes to food. Bmi is a surrogate measure of body fatness because it is a measure of excess weight rather than excess body fat. The positioning of your upper arm below your heart level will also result in higher measurements whereas positioning your upper arm above your heart level will give you lower measurements. Factors such as age sex ethnicity and muscle mass can influence the relationship between bmi and body fat. The measurements that you definitely need are your chest waist hips and inseam. But weight naturally ebbs and flows throughout the day based on a multitude of factors including hydration and bathroom patterns.
Do not use a metal measuring tape such as is used in construction it will be inaccurate. The process for getting your body measurements is simple. Ive been reading dr chrstine northrups book about womens bodies and she suggests there are 8 areas that take part so i thought id give my interpretation of why these factors are all essential to consider. You will want to use a soft cloth or flexible plasticrubber tape measure such as is used in sewing. Keep the feet together relax your muscles and stand up straight. You may also want to take your thigh and upper arm measurements.
The scale considers all body weight too. Consistency is essential measure under the same conditions and using the same equipment each time. These differences can increasedecrease your systolic blood pressure 2mmhg for every inch abovebelow your heart level. B waist measure the waist where the body bends. A scale is a common measure to track weight loss progress. Generally take measurements in an unflexed state and before your workout not after.
When taking body measurements you need to use the correct type of tape measure. It is better to wear fitted clothes when measuring. Absolutely no baggy clothes. It helps to bend side to side to identify exactly where to measure. Some general guidelines for taking body measurements the person being measured should stand straight on both feet feet some 15 cms apart. Measure all around the body total circumference.
This includes organs fluids bone muscles and fat.