The Kinase Glo Plus kit measures the amount of ATP remaining. It is capable of measuring ATP concentrations up to 100 µM, and the response is linear with respect to ATP concentration. I would advise against using this method for measuring the Km of an ATPase. Here is why. To measure the Km, you must measure the initial rate of ATP hydrolysis. This usually requires that only a small fraction (10% or less) of the substrate is consumed when the substrate concentration is not far above the Km, because the reaction slows down significantly once a substantial faction of the substrate is consumed. It is difficult to measure accurately such a small degree of substrate depletion because of the high background signal.

It is far better to measure the accumulation of the product (ADP) because the background is low, making it easy to detect a small amount of product. If you want to use a luminescence assay, you could use ADP Glo. Unfortunately, if you do that, you will have to prepare a different ADP standard curve for every ATP concentration, but that is not difficult.

You will have to collect ADP measurements at a range of evenly-spaced time points in order to be sure of measuring the initial rate of the reaction.

You might be able to use it if the amount consumed is enough to give a difference. In your case 3 to 100 micromolar ATP may be used.

You may have to use different reaction times for some concentrations but then you can normalize velocity. Linear range of reaction should be used in these cases.