Forecasting requires a lot of repeating entries. Right now, this is handled in Helm using Frequency. Frequency equals the number of days that pass between each transaction in your rule.
If you would like to setup a forecasting rule to create a transaction every month, Frequency = 30.
If you would like to create a transaction that occurs every two weeks, Frequency = 15.
If you don't want a forecasting rule to repeat (i.e. a single transaction on a single day), Frequency = 0.