I am creating a product where you can buy a shirt in custom quantities (3 smalls, 1 large, 4 mediums). The user types in the quantity, as well as select various product variations. Then by rules I calculate the price based on a fields I made called small_price, medium_price etc. So it would pull the users quantity input for each field and multiply accordingly e.g. small_price*small_quantity + medium_price*medium_quanitity. This is all working well, however it is currently implemented on the product level and not product display.
What I am trying to do is have the rule multiply a field from the product display by a field in the product variation. However the product display field is not available to me in rules. I have seen a couple of short hand answers on here about writing a custom rule but is there anyone who could explain it to me a little more in depth?