Guesses and estimates - refer to the Fair Tax research - the book is clear - economists who have studied the plan made that "estiamte" about the level of imbedded taxation.
As to pricing, compteitive forces do not change because taxes do - prices reflect competition. Where prices do not come down in order to reap exaggerated profits, compeitors will see the opportunity to gain market share by lower prices. Learn economics.
Look, they still make guesses and it's not like they have not been wrong before.