Can anyone help me make a formula for Healthy BMI Range based on this website?
Any help would be much appreciated!
You need to learn how your app can make decisions. Programming Your App to Make Decisions will show you some examples using If..then else Blocks that can be adapted to your Project.
What have you tried? Hint: you will need to decide which formulas you want your app to use.
You can make it precise. 
The World Health Organization's (WHO) recommended healthy BMI range is 18.5 - 25 for both males and females. Based on the BMI range, it is possible to find out a healthy weight for any given height. This means BMI's between 18.5 and 25 are considered healthy; all other values are not.
The BMI formula in metric units is:
BMI = kg/(m * m)
The WHO says that if BMI > 18.5 and BMI < 25 then the person's weight is considered healthy. They consider BMI's between 18.5 and 25 are healthy; other BMI's are not healthy.
For example, a person with a height of 1.8 meters and who weighs 65 kg has a BMI of 20.06
So you need some Blocks that figure this out.
You can use those Blocks to determine if a BMI is considered healthy. 
Be careful to use the right units.
Also look in the Gallery, there are many example of BMI apps there.