Printing WMS map with chosen scale and WMS zoom level using QGIS?

I am trying to print a WMS map in QGIS.

If, while in the print composer, I chose a "25000" scale, the composer chooses a WMS layer that actually shows a 1:50000 map. To get down deep at the 1:25000 WMS map in the print composer preview, I have to trick it by telling the composer to use a smaller scale, but then I don't get the scale I want.

How does QGIS choose its WMS zoom level?

I need to chose a scale AND a specific WMS zoom level. I tried the "Lock layers for map item" check box but it is not what I hoped for.