A dummy button can be useful to update your values with a function field.

Anyways you can write a on_change function. You can do something like this. I'm just supposing that your module is like this. But, It's going to guide you.

def onchange_amountPay(self, cr, uid, ids, amountPay, interestType)
interest=amountPay*interestType
return {'value': {'interest': interest}}

and on your view

<field name='amountPay' on_change="onchange_amountPay(amountPay, interestType)"/>
<field name='interestType' on_change="onchange_amountPay(amountPay, interestType)"/>

That it's going to help you. But it just calculate and show it only to the user. It's just to change it on the fly. The real value is calculated on the function.

If you have any question comment here.

Regards.