What is the minimum integer n to multiply the Michaelis constant of an enzyme to - (Nov/10/2015 )

I am using a NADP+ stock solution as my cofactor in an enzyme activity assay. The Km of this enzyme for the cofactor is about 2 mM. I just measured the concentration of my NADP+ stock solution to be a little over 4 mM. Would ~ 4 mM be an adequate saturating concentration for my enzyme as I varied the concentration of the enzyme's natural substrate? If not, how might the minimum saturating concentration be determined, if there is a secret rule of thumb for it? I don't want to use more NADP+ powder than necessary as it is too expensive. 

have you determined the saturation point for nadp of your enzyme? km is the substrate value at 1/2 maximal velocity, but 2 x km is not necessarily saturating.

This is what I wanted to calculate. Is there a formula to figure out the saturation point knowing the Km?

i've always run the complete saturation curve to determine the kinetic constants although you may be able to determine km and vmax by running a few reactions, with different concentrations of substrate, and prepare a lineweaver-burk plot. then, using the determined constants, you may be able to simulate the complete curve and determine the saturating concentration.

personally, i think it would be easier to run the reactions for the complete curve.

What if I solve for S with the equation v = 0.9Vmax, where 0.9 = S/(Km + S) and Km is known?

i don't think that will work.

solving for s becomes km/0.11111 or 9 x km (without knowing vmax).

i stand by my first comments.