You should never, ever do enzyme kinetics with endpoint methods, as you can only assume that the rate was constant between start and end. It is always better to measure continuously (doi:10.1111/j.1432-1033.1967.tb00078.x), or at least take several time points (http://www.jbc.org/content/244/23/6493.full.pdf?sid=2ad7cbe5-30d6-44f0-b305-87562fc5ee79). For low quantities of ATPase, one can also determine its K-dependent phosphatase activity with fluorogenic substrates (umbelliferyl phosphate, 3-O-methyl fluoresceine phosphate).

The calculation is simple, you divide the amount of product (ADP or Pi) produced by the time required, giving you the activity in mol/s = katal (that is a very large amount, usually you deal with nkat). If you divide the activity by the amount of enzyme responsible, you get the specific activity (kat/mg), which is a proxy for the purity of the enzyme.

"Biochemical Calculations" by Segel deals with the bread-and-butter calculations that constantly occur in the lab, despite its age it is still highly useful.

Thank you so much Engeulbert Buxbaum for your kind reply, it help me alot.