In Response To 'Re: build up a calculation program with variables'
If you are only going to try a modest number of different values for the variables then you can put all those variables together, perhaps at the top of the notebook, manually make changes, evaluate the notebook, see the result and repeat as needed.
If it makes sense to not give those variables any value and look at the result which will contain the variables and any numeric values you have used for other variables that is also acceptable.
If you want to make many changes to the value of the variables then there are other more complicated ways to allow changing the values.
I would suggest you begin simply. Perhaps start with very simple values for some of your variables. Test the notebook and check the results carefully to make certain that there are no errors. After you have confidence that the notebook is correct then it may be possible to make further changes to enhance it.