The one in twenty figure - or 5% - was a working figure for acceptable losses. The raid losses did not necessarily represent fatalities and many successfully bailed out. Where the loss rate on a particular raid was much higher than 5% then they became very worried.
The Augsberg raid was a case in point. Other raids early in the war with Wellingtons, Blenheims or Hampdens suffered much higher losses.
The thousand bomber raids however often had much lower loss rates as the concentration of force saturated the defences.
The final overall loss of life was around 50%.
A late friend of mine flew just one mission from which he failed to return. as a sgt nav it was his one and only Op and that the raid immediately following the inundation of the Ruhr and the predicted absence of flak, searchlights etc. Int had overlooked the fact that flak batteries used generators.