Ninja Forms free version – calculation error in preview

Hi all
Am designing order form for a catering company website (I’m not the developer), running Ambrosia theme (by Awe). Free Ninja Forms installed.

Set up part of form to test calculations and this didn’t work so followed @ozoneboy advice here: https://www.ads-software.com/support/topic/order-form-with-item-quantity?replies=3

But in preview, subtotal field does not calculate – no value appears.

My equation in subtotal is: (field_34 * field_20) + (field_35 * field_23) + (field_36 * field_24) + (field_37 * field_25) + (field_38 * field_26)

The fields involved in the calculation are Checkboxes for products with checked calc value 3.30 and unchecked 0; and Number fields with Default value None, for product quantity.

Some of the values in brackets will be 0 if people don’t select a product or quantity – i.e. 0 (Checkbox unchecked default value) x None (Number default value). Maybe this is an issue?

Failing that, any idea why calculation isn’t working? Anything else obvious I’m doing wrong like errors in the equation? Is it possible there is a problem with the theme requiring developer input?

Many thanks in advance, Alex.

https://www.ads-software.com/plugins/ninja-forms/