The answer may be that you can't: that it constructs a range of 5 values based on your current speed and the maximum allowable (1100 in your case, 900 in mine), in 75Mhz chunks.
I've looked around for the source code for the 'cpuspeed.so' plugin that 'configbutton' uses, but I can't find it. If anyone knows where it is, that would give you a definite answer.