There have been two releases of the Smoothie board, v1.0 and v1.1 and there are differences between them, the main one being the driver chips used. This impacts the available microstepping rates, and subsequently the E axis steps per mm that are used with a Flex3Drive.
The smoothie is an excellent board but does not allow adjustment of the microstepping. This is fixed at x16 on v1.0 and x32 on the V1.1 board.
If you are using a v1.0 it is possible to use the x16 stepping rate without any hardware changes, however with the v1.1 an off-board stepper driver module must be used (or cut the MS3 trace on the board).
The stepper driver module shown here available from RepRapWorld is easy to use with either board, and will permit microstepping at x8 by simple jumpers underneath the driver. More info about this is available on the Smoothie website.
Smoothie firmware is configured using a text file named config.txt. This is quite an extensive file, and so only the relevant settings are listed below. If you wish to fit one of the off-board stepper drivers, please contact us directly for help with re-assigning pins, and the subsequent values to change in the firmware.
Steps per mm for extruder stepper
E Axis max feedrate
E axis acceleration
Extruder max speed
E Axis motor current