Way to Use Calculated value based on Subscription Field value

Hello.

I was able to use the amount from a custom payment field and create a calculated value in a different payment field. I cannot figure out how to do the same with a subscription payment field. Is this possible? If so, what is the name attribute I need to use to get at that value?

ex.
custom payment field attribute name is payment_input

In a second payment field I have the following formula in the advanced>calculation expression:
{payment.payment_input}*.029+.30

So, an entered value of 20 in the first custom payment field yields 20.88 in the second payment field with the calculated value.

I also have a subscription field with a name attribute of recurring_fee. When I try the following formula in another payment field {payment.recurring_fee}*.029+.30 it does not work. It doesn’t pull in any value at all.

Thank you!