but I need to reference these fields on another related table
Is the key detail as it defines the needed "context" here. Define a calculation field in that "other related table" in one of two ways:
Sum ( YourTable::NumberFieldHere ) * 0.2 // multiple by 0.2 to get 20% of the total
YourTable::SummaryFieldHere * 0.2
When you refer to a summary field defined in table 1 from the context of table 2, you get a sum (or other aggregate value) based on the set of related records (all the records in Table 1 with the same ID as the ID of the current record in table 2). Sum uses the same relationship to get the same total and will update more smoothly if you are using a portal on the "related table" layout to edit these records in ways that will require that your computed value update to show a new total.
Thank you very much, works perfectly :)