
By '8 grey level images' you mean 8 items of 1bit images?
It does work(!), but it doesn't work if you have more than 1bit
in your screen and if the screen intensity is non-linear.

With 2 bit per pixel; there could be 1*c_1 + 4*c_2 timing,
this gives 16 levels, but they are linear if screen intensity is
linear.
With 1*c_1 + 2*c_2 it works, but we have to find the best
compinations -- there's 10 levels, but 16 choises; best 10 must be
chosen. Different compinations for the same level, varies a bit, but
the levels keeps their order.

Readers should verify what I wrote... :-)