Hi Jeff, so if I understand what you want correctly, you can just use the regular Transform > Rotate command to position your larger arc piece perpendicular to the curve.

To do this, go to the Top view, it should look like this:



Drag one of the large arc pieces and grab near the endpoint and snap it on to the curve where you want it, so it will look like this:



You can now use the Transform > Rotate command to rotate that curve to be perpendicular to the curve. The rotate command will use 3 picked points - for the first point for the rotation origin pick the end that you placed on the path curve. Then for the first reference point, snap that on to the opposite end of the arc, and for the end reference point snap that on to the perpendicular line off of the curve - note that you need to have "Straight snap" enabled in the bottom toolbar for that kind of snap to kick in.

So the 3 rotation picks look like this:




There's also a command Transform > Orient > Line to Line which kind of combines moving and rotating together in a convenient way, you could also use it for this job, some notes on how that works here:
http://moi3d.com/2.0/docs/moi_command_reference8.htm#orientlinetoline

Orient Line to Line also combines scaling of the result by the difference in length between the 2 lines that you pick, for a case like this you would want to disable scaling by setting the option for Scaling: None.


> Which is best?

I'm having some difficulty following along with what the alternatives are here, maybe this would make more sense if you could post the model with more of the pieces in place? The one that you posted seems to only have just the sweep-fillet piece alone.

- Michael