Calculating yardage is fairly simple math. Measure your cushions, figure out the panel pieces and how you will be sewing them together. Make a diagram of how the pieces would be laid out to cut them from the yardage. Add a little extra material and you have your answer. If you want to add welting/piping it gets a bit more complicated, but not much. FYI, matching piping (cut on the bias -45 degrees) takes a great deal more cloth than most think. Figuring yardage for patterned pieces is the complicated thought process.
"Good judgement comes from experience. Experience comes from bad judgment."
Sirs Gawain & Galahad
Air #48582, S/SO #003, WBCCI #4584