There's no petcock, on EFI bikes the "reserve" is typically the fuel level at which the low fuel light comes on. On our CT125 it would be the last bar of the fuel gauge blinking.
The owner's manual has some specifications on the remaining gasoline that validates this statement. There's also quite a bit of rounding going on for the 0.3 gal specification lol... Pg 21 from the owner's manual linked
here.
At some point I'll run the bike down to completely empty, but not before I strap a Rotopax on the back of the bike first. This bike is fairly light and easy to maneuver, but I live in a hilly area and would undoubtedly run out of fuel at the bottom of a hill, not the top.
Oh, and as for fuel MPG, I usually get somewhere in the 100-110 range, with 35-45 mph riding. If I'm 55mph'ing it it drops to around 93 mpg. I have a fuel dashboard linked in my signature with a running calculation of distance to empty, mpg, etc. That's at vehicle GVWR, on hilly terrain.