bumping holding current - how it affects torque?

[Jonathan]

Yes, the optimum ratio is a bit unwieldy. Not difficult with two belt reduction, if you've got the space. You imply that the requirement is 188W, plus a bit to account for the inefficiency of the gearing. If that's a planetry gearbox then you'll need to add quite a lot, but if it's timing belts then 200W is probably sufficient. You're probably not going to get that much power from a smaller stepper motor, but by all means check.

Where did the 37.1Nm figure come from? at 600RPM, you only have 2.7Nm torque delivered by the motor, so over a 6.66:1 ratio, there's 18Nm on the output. Or am I missing something?

No, you're correct. I made silly mistake and corrected it but you were too quick!