Here's one example of what you can do:
In SONAR set up a Bus you call "master". Send all the tracks and buses in your mix to you "master" bus.
Set the output of your "master" bus to the Focusrites output 3 and 4.
Hook up the RNC so the outputs 3 and 4 go to the RNC left right inputs.
Now, add a stereo track to your project and call it "mixdown". Set the inputs to a pair on the Focusrite. I'm going to pick 5 and 6 arbitraily for this example. Set the outputs of the "mixdown" track to your Focusrites "Monitor Outputs" a.k.a. output 1 and 2. (I'm assuming you are using the Focusrites monitor outputs for your monitoring.)
Hook up the RNC left and right outputs to your Focsurites 5 and 6 inputs.
Hit play and adjust the RNC.
You don't need to worry about any "insert" stuff because that is latency compensation for digital effects and stuff. Your RNC has an analog signal path so everything should be synced just fine.
If you discover that you have a sync issue, it is probably a global issue that has effected every over dub you have been doing... some people notice some people don't.
If you have to trace something like that down you'll want to do a round trip latency and loop back sync test... which is essentially what you are doing with the RNC patch anyway.
That should do it.
Have fun!