Never mind, I did some experimenting killing my storage mules.
Seems my base death loss is 5% (thought it was 10%??)
With Undead monster 'exp loss per death -2%', lost 4.9%
With Undead monster and S level Cannon Master card (-4%), lost 4.7%.
So the bonus for Monster life or CM character card is not x% off of the y% you would normally lose, but instead decreases what you would normally lose by x%. Probably wouldn't save a minutes worth of training in most cases.
Neither the undead monster life bonus or the Cannon master card bonus are worth bothering with.
Gosh Nexon, you are sooooooooooooooo generous.