Manually focusing from the screen is something thats going to be poor in that camera.
I wont go into all the specifics, but in "the old days" before AF, low/medium price SLR's were focus adjusted by tilting the mirror such that the center of the screen was "in focus", and thus making the focusing aid - such as the split image - work with good accuracy.
Better cameras, Nikon F series, Canon F1 series, etc... had their screens set into a holder that was shimmed at the factory (at considerable pain in the butt too I might add). The shimming allowed a perfect 45 degree mirror, which kept the image "square" (no tilts), and it also allowed focusing on any part of the screen.
The 1100D/T3i is not made with either of those benefits, so focusing off the screen is "catch as catch can..."