2 options I want multiplied not added together

I want to sell vinyl signs. You pick a size and amount of letters (there a text box to put your wording ) bigger letter are more expensive. So what I created was a text box where you put your text, dropdown for size (which go up in percentage), and dropdown for amount of letters again with percentage increase per letter(Presta shop had a option to do this automatically but csc hasent(yet ,unless you know of a mod).

The problem is if they have a 8 letter word and go for a large letter , it only adds the large letter once rather than the 8 times. I could add the letters as quantity but this way the product item price changes to reflect full price rather than waiting to add to cart button.

Is there a way to multiply the fields ?

Is there a text entered to total letter used mod so they wouldn’t have to select letter count? Prestashop has it