I'd be glad to find out otherwise, but I don't think there was any definitive 100% accurate calculation of last year's stay/play/dine discount.
I'd advise the rough way to estimate is to use hotel rack rate, ticket full price, but with the child price for all dining plans (adults or children). Using that method last year, my estimate was close enough (within $50) of the actual price of my S/P/D package.