How to accurately do Protein/Enzyme Assay Calculations?

This is for my Biochemistry of Macromolecules Lab.....
For determining enzyme concentration (umole/min) for M1, P1, P2, and S2 using the fact that 9OD=1 umole, does these calculations seem reasonable.
The OD's are: M1=.888, P1=.352, P2=.473, and S2= .034
I divided the OD's by nine to yield the umole units and then multiplied by the dilution factor 50 (20:1000) and .020 ml (amount of enzyme used)
To calculate the total units I multiplied the enzyme units by the volume in ml's from the protein assay from the previous week (it is total volume of the M1-S2).
To calculate the specific activity I divided the enzyme concentration by the protein concentration.
To calculate the purification factor, I divided the final specific activity by the initial specific activity and for the % yield of protein and enzyme I simply divided the final total units by the initial total units and multiplied by 100.......
HAS ANYONE DONE THIS BEFORE.....DOES THE CALCULATIONS SOUND RIGHT??