I have seen several equations in literature presented for the determination of enzyme activity. I have seen Actvity = Va-Vb x M x 1000 all divided by t x Vc; where Va =Volume (mL) of reagent in test titration; Vb= volume of reagent in blank titration; M = molar concentration of reagent; Vc = volume of sample used in reaction, t = time, 1000 dilution factor (a constant). I have also seen EA = ∆A×V×df÷ε×t×v where EA = enzyme activity (U/mL); ΔA = change in absorbance over time; V = total volume of reaction mixture (mL); df = dilution factor; ε = molar extinction coefficient at wavelength used (/mM/cm); t = incubation time (min); v = volume of enzyme in the assay mixture (mL) and many others. is there no way these equations can be standardized?
I think it is more important to understand what you are trying to calculate and how to go about it than to adhere to a strict equation. Also, different methods are used in different circumstances, so a single equation won't apply to everything.
By the way, if you are interested in the idea of standards for enzymology, have a look at STRENDA.
http://www.beilstein-institut.de/en/projects/strenda/guidelines
It is very easy to calculate enzyme activity. And everything is standard.
What is not standard is the method, i.e. each enzyme has its own method(s) for measuring enzyme activity.
Probably it would be good it you indicate whether you want to calculate parameters such as Km or Vmax, or you just want to compare enzyme activities of different preparations.
In any case the "standard" law is the Beer-Lambert equation; usually there is somebody in the institution who has done it before you. If you are the first you may need help from Research Gate of from somebody in your City who is familiar with enzyme activity calculation.
Beer Lambert law is:
delta Absorbance = epsilon x l (length of the measuring device) x delta concentration
You know that epsilon is (molar extinction coefficient) but for your enzyme the specific method and specific wavelength, do you know the numerical value of epsilon? This is the first thing to know.
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