There are four options to trigger Rules and apply Actions using Automation: 'On Creation', 'On Edit', 'On all Tasks Completed' and 'Budget Threshold Reached'

Fig 1 - Trigger Options

Select 'Budget Threshold Reached' and you will be presented with a field to stipulate your Budget Threshold.

Fig 2 - Budget Threshold Reached option

Here you can now specify at what point you wish an Action to trigger using an Absolute or Percentage amount of the Remaining Budget.

The 'Budget Threshold Reached' trigger will apply your rule when the Remaining Budget field updates and calculates the new Percentage or Absolute value.

These Remaining Budget field recalculates each time that you add/remove or edit Financial or Transaction records, so any alteration made to any of the records in these tabs could trigger this Rule and its Actions.

Once you have saved your Rule, then you can add Actions that you wish to trigger using the Action tab by pressing [New]. Here you can add any Action to trigger.

To send a Notification immediately to a specific Person or Role, you should add a Task (but any Action can be applied to trigger).

So your Rule could be:

'When Budget has reached 10% Threshold', create a <Task> to <Judy Weeks>.

If you do not add any Criteria to your Rule, it will apply to all of your Entity records. If you wish the Rules to apply in certain circumstances, then you must specify when the rule should apply using the Criteria options