If we convert the number into grams it makes the calculation easier...
27,890 mg = 27.890 g
27.890 g per 100 g of veggie capsule powder, that is 27.89%
27.89% of 900 mg (or 0.9g) is 251mg (0.251g) per capsule
Here is the calculation in algebraic terms...
27.890 g X
_________ = ________
100 g 0.9 g
Any way you slice it, the numbers in the paper don't seem to make sense. I think the real number is probably 25.1mg per capsule (as you said) and the scientists made a mistake in the paper by one decimal place. Thus the real number per 100 g of veggie powder is probably 2,789mg and not 27,890mg.
So if that is the case, in order to get the same effect out of capsules that you get in a large amount of arugula/spinach/beets, you'd need to take at least 10 -20 capsules. Like I said, they really caused me intense stomach pain so I'll pass on that, but if anyone wanted to try it, please let us know.