A band of 1mm to 4mm in 20m would be fine for long range outdoor lasers, as users are likely to generate more error variance by the way receivers and staffs they are used than by the tolerance of the laser itself. In specialised high accuracy engineering environment the tolerance of a laser becomes far more important.

Quoted Tolerances

The next points to cover; are the tolerance rating itself and whether the rating quoted has been checked post manufacture in the country of distribution. RedBack Lasers calibrate each and every laser level sold and then place a calibration certificate with date on the unit.  The maintenance of accuracy is determined by the quality of build and how it is used and stored by the user. RedBack’s are built to the highest standards and so under normal operating circumstances calibration will rarely drift significantly.

RedBack lasers calibrate lasers in Australia and make them well within their quoted tolerances.  This allows some margin for any minor drift and still remain well within tolerance.  Some other brands rely on the factory calibration before shipping from outside Australia.  They may also select a rating based more on marketing edge rather than reality.  I would suggest is quite prevalent on internet auction sites.

Most brands rate accuracy as a number millimeter over a number of meters such as 2mm over 20m or 3mm over 30m.  One brand I’m aware of quotes accuracy in arc seconds which for the average person makes it impossible to compare.  The reality for the brand in question is that when you convert their arc second figure over to mm per m its about the same as other comparable brands.

I hope this answers some questions on accuracy ratings on laser levels.