Hi Earl,
Here's a couple of attachments I used on this BBS once before - first one shows the chances of surviving a tour of X mission based on an overall loss rate of Y, second shows actual loss rates month by month during WWII for the bombers of the 8th Air Force and of RAF Bomber Command.
With the table, the number of missions is down the left, the % loss rate is across the top (it actually goes from high rates to low rates, I should really re-do it so the losses are low at the left and high to the right), and the % chance of survival is what appears in the cell.
Bottom line is, a majority of crews can expect to survive a 25-mission tour if loss rates are below 2.5%. (This is only of course operational losses, so with training accidents and the like one would want it to be a little lower.)
Fairly stark stuff for the 8th Air Force in 1943.