Formulae for Number Fields - interim results?

I have quite a large form laid out like a table in which users select quantities of various items.
The form calculates sub-totals (using quantity * price) then sums the sub-totals to give a grand total.

This works great on the front end, but I'm finding that the grand total values reported in my Notification emails are zero.

I'm guessing this is because the formula I'm using for the grand total field is the sum of sub-totals, which are themselves calculated using a formula.
Does Gravity Forms support using number fields as interim results in this way?
Is there some kind of issue with the order in which field values are calculated?

A Simplified Example of what I'm doing:

Field 1 Qty: Number
Field 1 Sub Total: Number; Formula: {Field 1 Qty} * 10

Field 2 Qty: Number
Field 2 Sub Total: Number; Formula: {Field 2 Qty} * 25

Grand Total: Number; Formula: {Field 1 Sub Total} + {Field 2 Sub Total}

The Grand Total field works on the front end, but gives zero in my Notification emails.