1. The label name should be inside ' not " i.e 'Total Due' not "Total Due"
2. Currently, the error message is only re-calculated if you visit the label. E.g on the above video when we add 0.13 to total tax, Total Due was till showing an error icon but once you move to total due and click the tab, an error icon is removed as validation matches. We are working on making all fields update in realtime, i.e as soon as any fields are updated all field effected by that change is recalculated and error message are added or removed. Should be live in 2 weeks as this required major change in the codebase.
3. If you need to use greater or less invalidation, please use >== and <==. For equal, you can use ==. The documentation will be ready next week.