I could be wrong, but my experience is NEVER attack at odds lower than 5:1 (as indicated by the numbers on the units). This seems to be somewhat confirmed by the high losses that you obtained for 2:1 and 3:1 odds. Even if the defender is practically annihilated, attacking losses of 20% or more for each attack will 1) rapidly whittle down your units to useless; 2) prevent them from getting any significant experience.
An interesting statistic would be the odds required WITH and WITHOUT mortars to ensure attacking losses less than 10%, which IMHO is what would normally be considered acceptable losses (does anyone remember the real number?). Correct me if I am wrong, but in WW2, for Western armies, losses of 30% were considered a terrible disaster ( the famous bombing attack on Germany that resulted in the complete stoppage of daylight bombing of Germany comes to mind - the Allies figured with those kinds of losses, they would soon have no bombers left to bomb Germany). The soviets with their almost unlimited manpower were a different story: as Halder noted in his diary, "...we destroy a dozen Soviet divisions, and they just replace them with another dozen."