To answer your first question, at least, the figure given is in 2017 US dollars.
I don't think any serious historian, or even moderately well-informed modern commentator, challenges the idea that Britain drained wealth from its Indian colony, or accepts the supposedly "mainstream" narrative that is mentioned on the site you link to – that is, that its Indian territories came at a net cost to the British empire and that British involvement in the sub-continent was an act of "benevolence" towards the native populations. So, frankly, I think the article itself sets up a bit of a straw man argument.
The book that Patnaik's essay appears in was published by Columbia University Press and there's no reason to suppose that it's anything other than a serious attempt to quantify something that – if only because early records are lacking, and the period under study is such a broad one – is essentially almost impossible to quantify. As you point out, however, the figure that Patnaik reaches is going to depend on a series of assumptions, some of which I'm sure may well be challenged by other economic historians.
Because her work first appeared less than two years ago, it's unfortunately still too early to expect much scholarly or critical feedback on her essay, and I think it would be very unwise for a non-specialist like me to attempt a critique. The only response I have been able to find to date is by Purendra Prasad, who is professor of sociology at the University of Hyderabad, and hence herself no specialist in the relevant area. This describes Patnaik's method without venturing comment on it. From The Book Review (India):
Utsa Patnaik discusses the drain of wealth during colonial rule highlighting three propositions: one, the concept of drain and its measurement as articulated by Naoroji and RC Dutt explained the fact that India’s global capitalist export surplus earnings were entirely appropriated by Britain by ‘paying’ local producers out of their own taxes which meant not paying them at all. Second, by the end of the 19th century, the drain became very large with India posting the second largest export surplus earnings in the world for at least four decades before Depression. Third, the gold and foreign exchange earnings thus appropriated from its colonies, especially from India, allowed Britain to export capital to develop Europe, North America and other White settlement regions ensuring rapid diffusion of capitalism in these regions.
