i starting to get some decking put down . now i have a large area i want to cover . the size off the decking will be 40 foot wide x 30 foot long . the thing is its over a slope of about 1 in 10 so there abouts . i have got 4 12 foot long x 7 inch posts coming for the back post . i thought off putting in 10 foot ones in the middle and maybe 6 foot at the front . the thing i want to know is how far do i need to dig them down and how much concrete to put in each hole .