Now you are on the right page. The steepness of the slope is known as the angle of repose. In a perfect world, you would have a geotechnical engineer and a surveyor come in, shoot some points and dig a test pit or bore out a core sample. He or she would give you a soil report and specify the minimum angle of repose. If this is not possible use the standard conservative estimate, one and a half times the depth. So 12x1.5=18 feet. Thats 18 feet back from the edges, not from the center. So if you want the bottom of the hole to be 16x18 the top, add 36' to each dimension, the top of your hole would need to be 52x54. If you only wanted the bottom of the hole to be 12x14, the top would have to be 48x50. These numbers are based on a depth of 12'. Now that you have some safe working dimensions, you can go out with some stakes and string and mark out your excavation. Measure corner to corner to ensure squareness.
So now lets figure out how much volume that is. For a 16x18x12 hole we figured out earlier 128 cu yds. (not including the angle of repose) If you remember from geometry class the area of a right triangle is 1.5(basexheight). 1.5(18x12)=324 sq feet. This is just the cross sectional area of your angle of repose. Multiply 324 sq ft by the number of lineal feet, which is simply the perimeter of the bottom of the hole, 68 (16+16+18+1
. 324x68=392 cu ft. Divide by 27 to get cu yd. Thats an extra 14.5 cu yds. Its actually more than that because i didnt account for the extra bits in the corners. Lets call it 18 cu yds for you angle of repose. Multiply by that by your swell factor of 1.25=22.5 cu yd. Add that to 128 and you are looking at a little of 150 cu yds of dirt to move.
There are formulas out there that you can apply your 150 cu yd volume to in order to figure out how long it will take you to dig the hole, but without knowing specifics of the excavator such as the bucket volume, and your personal cycle time (your skills/experience with an excavator) its just a shot in the dark. And that would just give you the amount of time it would take you to excavate, not getting rid of/spreading the dirt.