1-2 % deviation is a general rule of thumb. It could be is possible to drill as straight as down to 0.20% with the Wassara technology, but important to know is that this will require a complementary technology with active steering of the hammer. The active steering device is not a Wassara technology does not yet have but other suppliers have).
In brief, drilling is done and after every second meters, a monitoring device is dropped down to the position just behind the hammer where the hammers position is measured and identified. In case the position is according to plan then the drilling will continue. If not, the whole drill string is extracted from the hole and a bent sub including the monitoring device is mounted on back of the hammer and the hammer is put into action again. While continuously measuring the position and correcting the hammer drilling direction with the bent sub, the very high demands on hole straightness could be obtained.