Really it is hard, but it is no impossible.
You must forget all concept grow constant same, example: +1kg same +2cm in chest, waist and hips or varitions as +1cm chest, +2cm, +1.5cm hips. This is very imprecise.
You imagine sphere, this have a radius, diameter, circunference, area(a= π(r^2)) and volume(v=3/4(π)(r^3)), you suppose that you want know its circunference and volume, for example:
A sphere of 1cm radius = 6.28cm circunference and 2.35cm³ volume.
A sphere of 2cm radius = 18.84cm circunference and 18.84cm³ volume.
A sphere of 3cm radius = 12.56cm circunference and 84.82cm³ volume.
A sphere of 10cm radius = 62.83cm circunference and 235.61cm³ volume.
A sphere of 100cm radius = 628.31cm circunference and 23,561.94cm³ volume.
Okey you can see relation circunference-volume
Now you imagine that this is sphere is of aluminum its density is of 2.7g/cm³, How much does the sphere weigh in relation to its radius?
A sphere of 1cm radius = 6.28cm circunference, 2.35cm³ volume and 6.345g.
A sphere of 2cm radius = 18.84cm circunference, 18.84cm³ volume and 50.868g.
A sphere of 3cm radius = 12.56cm circunference, 84.82cm³ volume and 229.014g
A sphere of 10cm radius = 62.83cm circunference, 235.61cm³ volume and 636.147g.
A sphere of 100cm radius = 628.31cm circunference, 23,561.94cm³ volume and 63,617.238g or 63.617238kg.
All this is very similar than fat in body human, but obvius the human is not sphere of aluminum, but both have volume.
The relation volumen, weight and calories, this is 0.9kg of fat for liter, 7 calories = 1 gramo or 7000 calories =1kg for day, this calories only affect while is higher or lower than consume, for know the consume of calories exist multiples formules
in this page explain more of the formules and them use: https://www.calculator.net/calorie-calculator.html.
This is a example of relation of the volumen with bust, this is no 100% exact, but it can offer a idea. You can see than is more the increment of volumen between cup to cup, and diference of volume of cup 32A to 34A, both having same cup.
Chest, waist and hips weight distribution is diferent in relation them grow, based in the previus this is obvius, you should work now in diferent functions for chest, waist and hips, but not only that, so conisider thighs why the human body storage very fat, the arms can be optional, except that think in character very fat (morbid obesity or more).
Funtion chest and cup is not very hard of make with antecedents, but waist, hips and thights is some diferent, return at example of sphere of aluminium, but with the plus than the sphere have a core of iron of a 1cm of radius with its density 7.874g with a cover of aluminium 1cm of spesor.
A sphere of iron of 1cm radius = 6.28cm circunference, 2.35cm³ volume and 18.5039g.
A sphere of iron of 1cm radius with cover of aluminium 1cm of spesor = 8.84cm circunference, 18.84cm³ volume and 24.5789g.
Okey, this is similar human body with of bones and organs, for a person of 1.60m than 90kg and pass to 60kg no change its bones and organs, except the skin and muscle than change, it means that exist a weight invariable base and minum measurements of a person for than part of the body.
Minimum weight base, BMI (Body mass index)
Head = Head 2 BMI
Exqueleton = 3 BMI
Organs = 2.5 BMI
Blood = 2 BMI
Total 9.5 BMI
My personal plus to weight base
Min Fat. 2.5 BMI
Min. Muscular 3.5 BMI
Min. Skin 1.5 BMI
Subtotal 7.5 BMI
Total 17 BMI
Distribution of fat in diferents body parts, remember 1kg=7000 calories, this varition diferents type of body this is you should to distribute in multiples porcentajes of fat beetween bust, waist, hips and thighs, doing trial and error until achive to goal.
How make this functions of waist, hips and thighs,
this simple ,use table of varation weight and measuraments of a person of reference, I recommend this https://www.reddit.com/r/Malmalloy/comments/bs8gtu/update_on_mals_measurements_may_2019/
It is very complet in relation measuraments with the weigth include bust, waist, belly, hips, thighs, cup and bmi.
It should be noted that the density of the fat and muscle is not same,
Fat density: 0.9kg/L
Muscle density: 1.06gk/L
This responde question, because 2 persons with same tall, weight similar and with a type of body is not measurament same.