I am trying to set-up a field update rule, based on when a Potential 'Type' is created or edited - we make some assumptions about how much the potential might be worth from a list of survey questions. The categories of 'Type' are a direct association to our survey review.

When I reach the point of creating the field update on the workflow rule, the field I'd like to update is the Potential 'Amount', but this field cannot be selected from the list. 

Any help greatly appreciated!

Thanks!