Well in principle, if I get your question right, you could use any programming language for your application. You would create a script where you define some variables with values at the beginning, and then perform the calculations you want to so.
Might be a bit of overkill, but you can have look at Python(x,y). It's a collection of Python modules that can be installed easily, and comes with a nice user interface for the Python programming language, which might helps you getting started more easily. Plus, the Python programming language is very useful in many areas, so I think it's never a failure to have a look at it, anyways :-)
edit: I don't know if there is a portable version by the way, but at least Python can be run on many many platforms!