I just tested it. It is possible to do multiple castings on the same turn, but it doesn't kill 1/3 of population. It kills 1/5.
So you need 11 castings to get population below 10%.
Code:
pop remaining = (4/5)^no_of_castings * 100%
1 casting : pop remaining = 80%
2 castings: pop remaining = 64%
3 castings: pop remaining = 51%
4 castings: pop remaining = 41%
5 castings: pop remaining = 33%
6 castings: pop remaining = 26%
7 castings: pop remaining = 21%
8 castings: pop remaining = 17%
9 castings: pop remaining = 13%
10 castings: pop remaining = 11%
11 castings: pop remaining = 9%
Still it's hardly worth going over 3-4 castings, especially if you plan utterdark afterwards.