hi Ross, I can give it a shot.
According to the page you linked to, the intro says "you create a statement that uses data expressions and the names of existing fields" to build a calculated custom data field.
However, according to your calculation, you're just putting in the name of the form. So that's just not going to work at all -- you're never going to pull in the whole form in one go.
If you have one custom field on that form you'd like to pull in, you could go with something like this:
I hope that's enough to get you started.