its not the best practice to create a calculated field by calling this stored procedure

so this is my context : i have a rapport who work perfectly but i want to add a calculated field

formula of my field should be like this

if UNITE_COUT_MATERIEL = 'Unité' then (SUPERFICIE_INTER_CULTURE / SUPERFICIE_INTER) * QUANTITE_MATERIEL

if UNITE_COUT_MATERIEL = 'h' then (SUPERFICIE_INTER_CULTURE / SUPERFICIE_INTER) * DUREE_UTILISATION_MATERIEL

HOW CAN I PUT HIS IN THE FONCTION EXPRESSION DESIGNER OF MY DATASET ?