In the Preferences/Audio Devices make sure the 18i20 is selected in both the Input Drivers and Output drivers. 
The latency is shown in bottom section of the Audio Driver settings panel.  (I have a 6i6 2nd Gen - my backup unit - and my latency for a 64 buffer size is shown as 4.1 ms for both Input and Output. 
 
My Focusrite Control, which is probably the same as yours, is on defaults.   

Got it all working now.  Thank you.  One follow up question....
 
And I know this is probably an impossible question to answer, but what is a good "starting point" for Sample Rate and buffers?
 
I do mostly audio out of my FA08 and Moog, as well as some VSTis.
 
At 44.1 and 128 buffers, I get around 13.1 ms round trip. 
At 96 KhZ and 512 buffers, I get 50.7 ms round trip.
 
I don't have much experience, but are these in line with what I should expect?

I can't say.  I do mostly "analog" recording and low latency is a priority.  I usually record 24 bit/44.1Khz.  Both my main unit (MOTU 896mk3 Hybrid) and the 6i6 are set at 64 buffers.  You have basically the same PC as I have and I do not get any dropouts at 64 buffers and lower latency than what you show.  The biggest issue with latency, as I see it, is when you try to record with something that is already recorded. 

Thought I would chime in here. I have used both the UMC 1820 and the Scarlett 18i20. You can achieve better round trip latency with the Scarlett depending on your system. On my desktop I use a pci express card for USB audio devices, all the USB processing is handled on the card and there's no sharing of bandwith with other usb devices. This will go a long way in helping to get low buffer settings to work smoothly and dropout free.  With the Scarlett I was able to get the buffer to 32 and it was stable for me (but see below). The lowest the UMC 1820 will go is 64 but I really couldn't hear much of a difference when monitoring between the 2 at those settings. Both units sounded very similar, the Scarlett pres are a tad bit warmer to my ears, but I use several external preamps so that's a not an issue. The Scarlett is bigger and built like a tank, but the Behringer is not a cheap build at all the only real drawback being wall wart power supply. Both units handle ADAT without any issues. One thing the Behringer has that I like is each of the 8 channels have line/inst switches as well as pad switches. The Scarlett only has those options on the first 2 channels. The Behringer also has a knob to adjust between the direct signal and the input monitor signal from SONAR whereas the Scarlett only allows control of that in their software. Not a big deal either way, but the software with the Scarlett let's you set up a lot of nice mixing options for monitoring. The Behringer doesn't come close on that.
 
Unfortunately both me and one the other producer where I work had to return the Scarlett units. We both had issues with the devices losing connectivity in our DAWS and both of us had issues with odd noises coming from the unit when engaging the transport from time to time. He was using his on a MAC with Pro Tools, I was using it with SONAR on a PC. Not sure if these were just bad units or there were driver issues. So, while I still think the Scarlett is the better unit, the Behringers have performed better for us and the cost saved us some $$. Your results may (and probably will) vary.
 
 
 

Probably late for the OP ...
 
Through a twisty route, I recently acquired a UMC1820.  It's working very with SONAR for me.  I run it at 128 sample buffers and get sub 10ms RTL, which is perfect for my use.  I haven't bothered with trying 64 yet.
 
I like it and will be using it as my primary interface.

I do own a Behringer UMC404 which is a smaller version of the 1820. It works well with Sonar, the preamps are quiet and sound as good as you'd expect from Midas. 

Don't let bad impressions of 1st gen Behringer crap fool you, since they acquired Klark Teknik and Midas, their know how has grown considerably.  They still have some weak products but this sound card isn't one of them.

Go with the Scarlett 18i20.  It's more in price, but more durable and comes with additional software.