Spherical Mercator tile levels approximate scales for use in QGIS with GDAL TMS layers.

osm_scales.xml

<qgsScales version="1.0">
<!--Approximate Tile Scales for OSM and other Spherical Mercator Projections.
Source:http://www.3liz.com/blog/rldhont/index.php?post/2012/07/17/OpenStreetMap-Tiles-in-QGIS
-->

 <scale value="1:591659030"/>
 <scale value="1:295829515"/>
 <scale value="1:147914757"/>
 <scale value="1:73957378"/>
 <scale value="1:36978689"/>
 <scale value="1:18489344"/>
 <scale value="1:9244672"/>
 <scale value="1:4622336"/>
 <scale value="1:2311168"/>
 <scale value="1:1155584"/>
 <scale value="1:577792"/>
 <scale value="1:288896"/>
 <scale value="1:144448"/>
 <scale value="1:72224"/>
 <scale value="1:36112"/>
 <scale value="1:18056"/>
 <scale value="1:9028"/>
 <scale value="1:4514"/>
 <scale value="1:2257"/>
</qgsScales>