How i Create auto Substract Function?

[Mark]

Unfortunately not possible anymore and no alternative is being offered. Read the Caution at the end of this Guide.

You could still use a custom theme to obtain the desired result although custom themes are now under Obsolete Features and could disappear shortly. Read this post by the developer:

Equation into a custom field & sum into another custom field

The latest version (18.7.64) allows to set custom field to number type. This allows to do these calculations using custom themes. Let’s say you have two custom fields: Field 1 Field 2 and you want to show on invoice Field 3 which is total of Field 1 and Field 2 All you have to do is inject following lines to your custom theme: <div style="font-weight: bold; padding-top: 20px">Field 1 + Field 2</div> <div>{{ custom_fields["Field 1"] | plus: custom_fields["Field 2"] }}</div> The most import…

Note: i.s.o. plus you use minus.