Briter,
Although I'm not taking as high a dose of oxy as you - 20mg 3x day - I wanted to let you know that it does NOT last the whole 12 hours...for me it usually lasts about 6 hours. I take one at 5 am, one at around 11-12 depending on pain level, then the last one around 5 pm.
I also take vicodin 5/500 1 1/2 in between for b/t pain. The pain never goes away.
I too sit in a chair all day and you're right...it's no picnic. Today I'm feeling exta bad

It's one of those really bad pain days.
My pain is in my stomach and I use a heating pad almost all day...seems to help a bit.
Anyway, just wanted to reasure you that oxy's do not last 12 hours.
Have as pain free a day as possible
Angela