What is equivalent to 0 VU meter level in Sonar dBbfs meters?

The other thing which is not considered here yet is the interface itself. All interfaces produce a maximum output voltage (rms) for 0 dBFS. Yours produces +10 dBu at 0 dB FS. Mine produces +18 dBu for 0dB FS.  A top of the line RME might put out +24 dBu or more at 0 dB FS.  (by the way this is where serious mains powered interfaces actually do much better with proper power supplies in them. Typical USB powered devices are only likely to reach +4 dBu at 0dB FS) Can you see here that the obvious ref level for the RME should be -20 dBFS. Because at that ref level it is going to put out +4 dBu which is serious!)
 
My Yamaha digital mixer puts out +18 dBu at 0 dBFS so at +4 dBu I need to running my digital ref down at -14 which I do a lot for most things.
 
Of course you can still set any ref level you like to be 0 dB VU but what level that actually comes out of your interface at that ref level will vary. So If I want to work say at -20 which I like because of the K System ref then it means that I am only going to get  -2 dBu at that level which is still OK.
 
For your Scarlett interface max output is +10 dBu at 0dB FS. If you wanted to produce +4 dBu at your ref level then your ref level would have to be -6 dBFS which is not great obviously. Lets take your ref level down to say -18 dB FS. That will work fine too but you will only get -8 dBu at that ref level. (which is very close to -10 dBv by the way) 
 
All good as long as you realise that at your-18 dB FS ref level you are only putting out that amount of voltage. The next question is it going to be enough to drive a pair of active monitors. I would say yes because with most active monitors when you turn them up as high as they will go the input sensitivity will be around -10 or so. The volume controls should work over a range of input voltage levels and that is the very reason they give you that facility.
 
So if you wanted to produce 85 dB SPL in your room you might have to crank your speakers up to the -10 input sensitivity. When I am producing 85 dB SPL in my room I might have my speakers set around +4 dBu sensitivity. (for -14 ref or -2 dBu for -20) We are both producing the same amount of SPL but you are adding in more gain within your monitors in order to do it.
 
To cut along story short -18 is a known good ref. I like -14/-20 only because in Studio One they have K system meters which can be set to either of those. -14 is good for me also because my Yamaha digital mixer all tallies up at -14 with everything too. If I know I am going to be producing a loudish master at the end of the day I may start at -14 because then it is not too hard to master up to higher levels such as -10 or higher even. -14 is great level because it is quite loud and punchy but will retains nice dynamics and transients. -20 is better still though for transients and dynamics.