What pulley position to choose?
That's literally impossible to properly give any accurate answer for since it depends on just what your doing. I look at the material I need to machine, and with a mill, then the maximum and minimum cutting tool diameters I'm going to be using as the first two priorities. Then pick a belt / pulley position where I'm still going to be in the general ballpark of what I'd be using without the VFD and using a single ph machine. Even then, small or large diameter drills, end mills, face mills, fly cutters, power tapping, larger bored holes will still dictate when your going to have to shift that belt or use back gear. And fwiw, Bridgeport are cautious about listing in the documentation I've found on line about limiting the maximum spindle rpm to about 500 when you are using that back gear. With a new to you mill and looking for a VFD to power it, some forget that back gear also requires the motor to be run in reverse to then have the correct spindle rotation. It's less common today, but not all VFD's and especially the cheaper one's have that reverse option built in. So it's at least something to check before buying.
With an Asian sourced BP clone and what many don't seem to know. Then almost for sure it was also built to an Asian motor standard that doesn't conform to any of the more usual standards for Imperial or Metric frame off the shelf motors. That was pointed out by my industrial electrical supplier when I was buying my VFD. He also mentioned that "some" non VFD rated motors and it's apparently fairly rare, will refuse to play nice when powered with a VFD and can sometimes burn themselves out. My best guess is that probably happens when corners were cut on the amount and type of motor insullation that the motor manufacturer did. So just in case, I choose to vary my VFD by at most 30 Hz on either side of the motors rated rpm and change the belt position whenever I'd need to go outside that. It litterly takes less than 1 minute to change to a different pulley ratio, so I don't understand why anyone wouldn't do so. I'm using what was a quite expensive industrial rated 5 hp Baldor VFD on my mill, and even with it, the VFD would sometimes fault out in the top two belt speeds simply because the torque was a bit too high to get the spindle up to full rpm in the ramp up time the VFD was factory set for. Changing the parameters and adding 2 seconds to that ramp up time permanently fixed the only minor issue I've ever had with it.
I also have my VFD's hard wired into a seperate sub panel in my shop and an emmergency stop wired between that sub panel and each VFD . Unless I'm actualy using a machine, or when changing tools, those E stops are used to turn the supply power off to each one. And I'll trip the breakers for the whole shop anytime I going to be away from it for any length of time. In my opinion, buying any non UL compliant VFD just because it's dirt cheap is taking a massive chance if it ever did start a fire. You can virtually guarantee your home insurance would be worthless if any fire was ever traced back to it.
