Determining flex shaft speed

I just received my Wecheer 1/2 hp tool with the Wecheer tabletop dial speed control.

I'm not sure how to determine what the no-load speed is for any given point on the dial and I'm concerned about over-revving the reciprocating tool handle. From what I've read this can quickly ruin a handle.

I can't find much on this handle in particular but I think the Foredom has a maximum limit of 8000 rpm. Even if this is right, how do I know when I'm in this range? The Wecheer website is leaves a lot to be desired.

Is the dial a strict linear speed progression as in is the dial at full rotation equal to full speed and at half rotation equal to half speed? My motor's max is 30,000 rpm.

Ant insight would be appreciated.