If max contrast is 75, I'd assume that 50 percent or less would be 37 or less assuming you are set to 100% screen brightness in the pdp saver menu. If you go by the Panasonic whitepaper, 50 percent of max is the recc. for the first 100 hours. However, no rule is absolute and I want to enjoy my TV now. My preferred setting for contrast is around 48 which gives me the best PQ to my eye so that's where I set it. I simply fill the screen almost all the time and am being careful about high contrast logos.
I use the screen wiper just for added insurance for times (like now) where my kids are downstairs with their mother watching Noggin. It's in "stadium" mode but Noggin has an aggravating bright logo in the bottom right corner so I also have the screen wiper on. The kids don't seem to mind a bit.