You do not specify who's DSP or which model you have but for a RichAuto A11 you can set it up for spindle speed under G-Code control. In the DSP settings you first have to enable the reading of "S" in Auto Pro Setup -> G-Code -> S Read. By default it is set to ignore. Then you find the spindle speed table, (it's in Machine Setup, Spindle Setup, Spindle State) there are settings for the RPM values of 1 to 8 as you note. In these settings, set the RPM values to correspond to the spindle speed settings of 1 to 8 where 1 = 0 and 8 = Max.
You can still override the running spindle RPM using the manual speed control buttons on the DSP while your program is running.
Have mine set to:
8 = 24000
7 = 20000
6 = 16000
5 = 12000
4 = 9000
3 = 6000
2 = 1000
1 = 0
There is essentially no difference between 2 and 3 because my spindles min = 6000 but...