Multiply two field into a custom field

Hey, I’m trying to implement a field, which multiplies two other fields, but I’m a php noob. ??
I want to multiply the woocommerce order item quantity with a custom product field called real_quantity. The new field should be named calc_quantity. I already created the calc_quantity custom calculated field as described here: https://docs.algolplus.com/algol_order_export/fields/

Can someone help me with that?
Best
Felix

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