Potentials - Adding Custom Fields

Potentials - Adding Custom Fields

I have created a custom field that takes the AMOUNT of a Potential and multiplies the field by 40%. Example:  ${Potentials.Amount}*0.40

I would like to know if it's possible to add this field in a " bucket amount" that can be used in another formula. For example: Let's say that a Potential has field for the custom field ${Potentials.Amount}*0.40 that equals $100. A separate Potential has this same field (${Potentials.Amount}*0.40) that equals $200. 

What formula / function can I use that will allow me to add the $100 + $200 even though they are in separate Potentials? 

Thanks.