The best measure of nutrient sufficiency is always enzymatic activity of the most demanding system or the compartment that gets last dibs on the nutrient, provided nourishing that system does not create problems elsewhere.
But one is always in search of simple, quick and rough measures of the sufficiency of vitamins and minerals in a dietary.
Consider this possibility:
The liver gets first dibs on all nutrients absorbed or recycled into the enterohepatic circulation.
Liver absorbs vitamin K1 and vitamin K2 from the enterohepatic circulation and provides vitamin K1 to the rest of the body (which the body converts into the K2 with 4 isoprene units as needed, executes the vitamin K cycle for a certain period, and then metabolizes and excretes the metabolites) and absorbs vitamin K2 vitamers (including high molecular weight K2s made by intestinal bacteria) until it is satiated. As vitamin K2 continues to be absorbed, albeit probably at a lower rate, does the liver then begin to release excess K2? If so, K2 status in plasma could be a rough measure of sufficient vitamin K status in liver.
Is the same true of choline, taurine, and carnitine and many others? The liver must be satiated first with at least a minimum functional amount before the rest of the body is nourished with these nutrients.
However, it is likely a bit more complicated. For example, even though carnitine, choline and taurine can be made from methionine, dietary carnitine, choline and taurine are still highly sought after. Like the liver, the heart has a strong demand for carnitine, and it would not be surprising that some dietary carnitine is released for uptake by the heart even when the liver is still hungering for more dietary carnitine, and the muscles get nourished with the leftovers of this ingested nutrient, forced to try to make do with what they can synthesize from methionine stores.