what procedure u are following depends on the definition of unit activity
The FPU filterpaper unit is amount of enzyme capable of releasing 1 micromole reducing sugar (as glucose) per 1 ml per 1 min under standard assay conditions. Note that releasing 1 micro mole of glucose is equivalent to cleaving i glycosidic linkage in the substrate
As others prefer to to define FPU unit as the amount of cleaved glycosidic linkages in the substrate per ml per min under assay conditions