At the moment, once you create a multi-source widget with specific metric, you are able to use it within the original report where formula was created and also on other on-demand reports. Since the multi-source widget functionality has few limitations at the moment, we would highly recommend for you to keep the multi-source widget and the metric formula in a separate on-demand report that the formula would not be deleted. Also, please note that once formulas are saved in an on-demand report you can use them for automated reports as well.
Please see the case scenarios when the formula is no longer available:
The report gets deleted where the multi-source widget and the metric was created;
The report was automated;
Please see videos below to see how it works in practice:
1. Saving your custom multi-source widget formulas in the report
2. Case when the formula becomes unavailable