Source Expression formulas

I've just added a formula to the sales line subform to display the percentage of each line which has been billed.

If I want to not display anything for lines which have quantity=0 I could change my function to return a string and simply return a blank string, but is there a better way. Can I set a format on a cell so that it displays nothing when the value is zero?

Has anyone seen any documentation on what you can put into the sourceexpr field - I know you can put some formulas, but any old bit of c/al is not accepted so there must be some docs which say what is accepted. I've searched what I've got of the nav 5.0 dev docs and can't find anything.

Ian