We've made a lot of cool changes in FinalBuilder 3 with variables.
This particular enhancement makes it easier to edit expressions in the action properties dialog.
When you're editing any field in the action properties dialog, you can press Ctrl-Space to display a hint of what the expression evaluates to. Also, when you type “%“ the hint shows automatically.
This next screen pic shows what happens if the expression is invalid:
And hitting F3 allows you to add a new FB variable without closing the action properties dialog :)