Hi Burr - that's kind of an uncommon thing in CAD programs (or just drafting in general as far as I can remember anyway) to define an arc by its length rather than its angle.

So you'll need to use a formula to do it.

For the formula you're going to need to know the desired radius and length and then calculate what angle to use from those.

Let's see - the length of an arc goes like this:

Length = Angle_in_radians * Radius

So then we want to re-arrange that to be like this:

Angle_in_radians = Length / Radius

Then the input to MoI will be in degrees so you have to convert from radians to degrees:

Angle_in_degrees = (Length * 180) / (Radius * PI)

So you type that in for the angle value in MoI's Arc center command, replacing "Length" and "Radius" with your actual length and radius values. You can hit the Tab key to put keyboard focus in that Angle box at that stage of the arc command.

- Michael