I have few doubts in calculating enzyme activity.

I have plotted standard graph using Maltose(0.5mM to 3mM)
now i have to calculate enzyme activity of amylase from saliva using starch (5%)as substrate with DNS reagent (dinitro salicylic acid). I have taken O.D for 3 min( i did not take O.D at 0 min.)

so my question is for calculating Molar absorption coefficient do i need to take concentration from each min. (back calculated from O.D) Or concentration obtained from the difference of O.D (O.D of 1st min - O.D of 2nd min)

I would like to know the formula to calculate enzyme activity.

I have found on formula for enzyme activity,
delta Absorbance X total enzyme assay volume X dilution factor
U/ml= --------------------------------------------------------------------------------------
time of assay in min. X Molar absorption coefficient X volume of enzyme extract

I have doubts in this also,
total enzyme assay volume - means what do i need to take? is it only substrate and enzyme volume or do i need to include volume of DNS reagent added and water that i added before taking O.D.

Delta absorbance - means is it O.D of each min or O.D difference of 1st and 2nd min

time of assay in min. - means is it 1min or 3min