Hi Tony,
You are looking for in the region of 500:1 to 1000:1 for the best speed vs torque performance.
The setup I have is an EQ5 (144:1) with a 2.666:1 pulley giving 384:1 which is on the low end, but works well.
The Skywatcher DA kit motors have a 120:1 ratio which when coupled with a 144:1 EQ5 gives 17280:1 total which is very high. Gives brilliant torque, but max speed is around 40x the sidereal rate before the motors can't physically spin any faster.
With a 400:1 gearbox and your 21.666:1 friction gear you end up with about 8400:1 which is quite large. Depending on how fast your motors can spin that may give you decent performance - a Nema 17 motor will probably have far better top speed than the crappy motors in the DA kit. It's certainly worth a try. The other advantage of the higher gear ratio is that microstepping doesn't need to be used which will give the motor a bit more oomph - you can go with 4:1 or 8:1 microstepping instead of 32:1.
You could try and see if you can modify the gearbox down a bit - say to 100:1 or so, or stick with 400:1 and see how it goes.
