I'm trying to determine the outside diameter of the attached part, or at least close to it.

It's going in a 36" diameter Cylinder, but the diameter of the attached part should be a little more than that I think...

Is there a way to get this diameter?

Chris,

I created a sketch on the right plane, placed a horizontal construction line with each end coincident with an edge, and two equal length vertical lines going both ways from it's centerline. That fully defined the sketch, so the dimension shown below is driven. If I zoom way in it seems like the flight isn't quite normal to the plane, and so the upper and right sketch points are coincident with the inner edges instead of the outer, but I couldn't get them to lock onto the outside edges, so I'm not real sure what's going on there. Anyway, this should be real close.