Just to point out that the lens is 40mm, not 35mm. Stick a bit of white tape on the lower part of back of the camera and, with the aid of a tape measure, mark it as a human rangefinder* if you need reassurance over focussing. In general, thinking of the width of your table, the length of your bed, the length of a room in your house etc. will be good enough.
*Hold the camera at arms length -- close your right eye -- line up the left of the camera with part of your subject -- swap eyes -- you will see the subject 'move' along the camera. With help of the tape measure, make marks at the intervals written on your lens barrel (0,9 1 1,2 1,5 2 3 6 metres on the Sonnar), or at least until you run out of camera width. The spacings of the marks you make on the tape depend on the separation of your eyes and the length of your arms, so they are not really universal, but that will cover the nearer distances for you.