Unlike most kitchen sinks, which drop into a hole cut in the countertop and are supported by the countertop and cabinets below, an apron-front sink slides into an opening or gap in a countertop and is supported by the base cabinet only. A large porcelain sink can weigh 300 pounds when filled with water, so supports must be added to the cabinet to carry this weight.
Measure the "true" height of the sink, from where its top aligns with the countertop surface to the bottom of the sink’s sides, which will rest on the supports.
Transfer this measurement to the interior cabinet sides and mark a level line from back to front. This is where the supports will be added to carry the weight of the sink.