My solution below is in Inventor 2013, but since you didn't tell us what version you're using, I don't know if you can open it.
This isn't as easy as it ought to be, but here's how I did it (see attached). I created a view sketch, projected the two arcs, and created an arc concentric with the big "circle" as an extension to show the complete circle (set to the Dimension layer). I then put in a Sketch-Only line between the arc centers, then a normal line tanget to the sketched arc and perpendicular to the previous line. This last line is purely for something to dimension to. I exited the sketch, then created a dimension between the tangent line just placed and the quadrant of the small arc. Then I went back into the sketch and changed that tangent line to Sketch Only so it is not visible.
Sam B
Inventor 2012 Certified Professional
Please click "Accept as Solution" if this response answers your question.
-------------------------------------------------------------------------------------
Inventor Professional 2013 SP1.1 Update 1
Windows XP Pro 32-bit, SP3
HP EliteBook 8730w; 4 GB RAM; Core™ 2 Duo T9400 2.53 GHz; Quadro FX2700M
SpaceExplorer/SpaceNavigator NB, driver 3.7.18
still waiting for a foreshortened radius dimensioning tool in Drawing Manager
