Can't seem to get -level to work

Hi,

I don't seem to understand the parameters of -level, or there is a bug?

I want to adjust the black and white points by about 10%.

Please see this image for the input and output of the parameters I'm 
trying (I've also tried percentages, floating point, exchanging the 
values, all to no avail):

http://imgur.com/94NjcT0

Thanks for any advice,
--Toby

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