I think I should also add at this point. The goal of the refund assessment tool is to make the accountant's job easier.
On the development roadmap, I'm going add a feature that lists everything a user might need to use the tool (or file their return) eg. payslips, bank statements, medical bills etc.
There'd be no point in collecting all this info and doing the assessment only to have to repeat the whole process again later.
The user will be given the option to download a pdf printout of all the calculated figures (including formulae) to show their accountant or do their own return.
I might charge a small fee for this part...