I think the issue here is that it is, as DougDeacon states, more accurate to have the appropriate angles to the groove at the correct angles to the record surface. In practice this may or may not be when the tonearm is level. While many, including myself, may start with the arm level this position is based on an assumption that the 'table, arm and cartridge manufacturers all are exact in their manufacturing processes. Most probably not the case, and that's where fine tuning the tonearm comes in. This last part is not at all easy with the stock Rega mounting.
