How i Create auto Substract Function? Substraction issue

Hello Everyone
I have 2 custom fields Start Counter and End Counter. I need an auto-to-subtract function in Quantity Column. It is possible for me I have just added the start and End counters in the custom field and they can auto-subtract the amount in the Quantity field.

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:

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

I may add that even if themes were not in obsolete features, you still cannot pass any of your calculation as inputs in other fields.

So, you would still have to fill your Qty manually or use in-field calculations like: 100 - 30 inside of your quantity field.