ATG’s documentation, as far as I can tell, does not document how to specify the calculator for a promotion. Promotion’s have a repository property called pricingCalculatorService. In the ACC you won’t see it by default but you will see it if you show expert-level information. Also you can see it configured in the Promotion repository (/atg/commerce/pricing/Promotions).
Having a repository property for each promotion allows you to specify different calculators for different promotions. But typically you will probably specify the same one for the same class of promotion, e.g. /betweengo/commerce/pricing/calculators/ItemDiscountCalculator.