Streaming Newsflashes use Salesforce's "Streaming API" to track changes on individual records based on customized criteria. Unfortunately, Salesforce does not support formula fields in the queries involved with each "Push Topic".
If you are trying to use a formula field in your Streaming Newsflash criteria, you will get an error because it is not supported. However, you can work around this by creating a new field on your object, which is updated by a Salesforce workflow. Here's how!
Create a Field Updated by a Workflow
In Salesforce Setup, navigate to App Setup > Customize and select the object upon which your Streaming Newsflash is based.
Click Fields to see a list of fields on your object.
Click New to create a new field.
Specify the field type. For example, select Currency if the number you're seeking to use in your criteria is a currency.
Give the field a label, and save.
Create a Workflow Rule
Navigate to Create > Workflow & Approvals > Workflow Rules.
Click New Rule.
Select the object upon which your Streaming Newsflash is based.
Give the rule a name.
Specify the rule criteria. Example: if you want to update a certain value when a deal is won, add "Won equals True".
Save your new rule.
Add a Workflow Action
After creating your workflow rule, you can add your workflow action.
Click Add Workflow Action and New Field Update.
Give it a name, and select the field to update.
For options, select "Use a formula to set the new value".
Enter the formula that gives you the value you need. This may be copied from the formula field you originally wanted to use in your Newsflash criteria.
Save your changes.