Whether the data access is abstracted at the page level (Tim’s post) or the container level (my post), you should not have Session methods that directly access the controls to get their data.
Of course having disagreed with Tim about page vs. container in general, I find myself wondering if part of the problem is that you have code in Session which really belongs in your page subclass. I think he may have nailed it in regard to this specific instance. Getting form data, running calculations, and displaying the results doesn’t sound like code that belongs in the Session class.