Solver With Code-Generated Model

Suppose that you have some data points and you want to fit a straight line to them. You write the line's equation in the cells ajacent to your data points, make the square of each difference, sum up all the squares and you have your objective function. Of course your line equation depends on two parameters, that you enter in two cells. Then you run the Excel Solver, enter your objective function as target cell to minimize, enter the range containing your parameters, set your constraints and off you go.
So far so good.

And now suppose that in order to generate your model data it is not enough to write a straight line equation in some cells, but rather to run a complete reactor model (contained in a macro). How do I couple that model-generating macro with the Excel Solver? Now, I actually found a solution. I wrote a Worksheet_change macro in the sheet containing the model parameters, which calls the model-generating code each time the parameters are chenged by the Excel Solver. This has worked fine for a simple example as the straight line one.

Now one of the two:.....