When pulling and pushing in a rapid sequence, the variometer may well deviate downwards but not upwards over the zero line. The remedy for these problems is to mount the probe further ahead of the horizontal tailplane with a longer probe that is less sensitive to angle of attack. The head of the probe should be exactly in line with the plane of symmetry of the tailplane. The fuselage creates a very strong pressure field around itself which will seriously disturb any pressure probe in its vicinity. This is even so in clean straight flight without any side-slip.
Conditions become even worse during side slips or accelerated flight.
An estimation of errors created is quite difficult because the airstream pattern around the fuselage is very complex and naturally different for every type of glider. Avoid the area near the wing. The further away, the better. The measuring head of the probe should be as far away from the fuselage as possible. As a general rule of thumb, at least a full diameter of the fuselage at the mounting point.