Updating my formula

Hello Ada Napierala, 

You could try including this in an add() formula with your custom fields. 
Something like this for a custom field named custom_field1_c: 

add(rollupConditionalSum($revenuelineitems,"likely_case","sales_stage",forecastSalesStages(true,false)),related($revenuelineitems,"custom_field1_c"))

Hello Ada Napierala, 

You could try including this in an add() formula with your custom fields. 
Something like this for a custom field named custom_field1_c: 

add(rollupConditionalSum($revenuelineitems,"likely_case","sales_stage",forecastSalesStages(true,false)),related($revenuelineitems,"custom_field1_c"))

Thank you Andre :)