Actually, it's not a drop of blood, it's a drop of blood for the rest of your life. But under a reasonable interpretation, Quirrel is perhaps being a little paranoid in avoiding use of that spell.
If we interpret the requirement as it frustrates one drop of blood from coming into creation, well, blood lasts ~120 days; if one drop is 0.05 ml and Quirrel is middle-aged and can expect another 40 years of life (the question about wizard lifespans is relevant here, though), then that's 0.05 * (365/120) * 40 = 6.1 milliliters total loss.
Or if we interpret it as reducing the total capacity of one's blood, well, adults have ~5 liters or 5000 milliliters, so you could use that spell hundreds of times before appreciably reducing your blood content (200 * 0.05ml = 10ml, so you'd go from 5000 to 4990...).
The latter explanation was my assumption. I am curious whether this capacity loss transfers across bodies when one is possessing someone else or has been resurrected.
This is a new thread to discuss Eliezer Yudkowsky’s Harry Potter and the Methods of Rationality and anything related to it. This thread is intended for discussing chapter 90. The previous thread has passed 750 comments.
There is now a site dedicated to the story at hpmor.com, which is now the place to go to find the authors notes and all sorts of other goodies. AdeleneDawner has kept an archive of Author’s Notes. (This goes up to the notes for chapter 76, and is now not updating. The authors notes from chapter 77 onwards are on hpmor.com.)
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17,18,19.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically: