According to mwsmith2's BleachCalc program you need 13 pounds to get you to 80 ppm. I would put in a 4 lb box into the skimmer and then test 24 hours later and as you get closer to your target I would add less and less and retest 24 hours later. (It take a while for the carbonate/bicarbonate buffer system to stabilze in the water. With your TA at 157 your CH is pretty low most likely. You can probably run as high as 120 ppm TA with no problems. I would shoot for around 100 to 110 ppm.

If you haven't already done so download BleachCalc....most of us on here use it!
http://home.earthlink.net/~mwsmith70/data/
then click on the link for bleachcalc262.exe to download it.