The best way to get a raster for printer with 3 levels per pixel?

[Paul Newall]

It seems my (kodak ESP) printer can make 2 dot sizes. So it can produce dots for each colour that are: white ,small, or large. It has 3 levels per pixel per colour.
The data to be sent to the printer has 2 bits per pixel per colour. 00=white, 01=small dot, 10=large dot. If you send 11 you get white.
So I am using a raster with 2 bits per colour, which will have 4 levels 00,01,10,11 and mapping the result to get 3 levels, so 00=00, 01=01 small dot, 10=01 small dot, 11=10 large dot.
This is not very linear so I don't know if it gives the best result. Does anyone know of any better ways to produce the 3 level output?