For anyone's reference in future this is the setting:
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Initially I set the spindle min to 5000, max 24000 which I assumed meant it would just not allow a command below S5000, but figured out that meant 5000rpm = 0v, so when I set 10000rpm I would get 7500ish instead! So set min to 1, max 24000 and all good (to the nearest few hundred anyway).

Playing around in UCCNC reminded me of a couple of other things
- is it worth setting max velocity and acceleration at this point, set to 2000 units, so 20m/m which I expect is faster then it will go.
- Is there any worthwhile tuning of steppers needed - these are the hybrid steppers so slightly less common. I see there are 4 switches which can be used to set pulse/rev but no idea if these are worth changing?
Or maybe I'm just thinking up ideas while waiting on parts:) Feels like I'm on the home stretch now, just need to figure out my workholding and spoil-board which doesn't eat up the Z distance.
I also couldn't find an obvious answer on this - can I use normal 1/2" router bits in the spindle or does everyone use endmills? Obviously something like a Lock Mitre bit would be a bad idea, but the simple 2 flute straight bits, chamfer etc..