I use the Diamond Racing Pistons calculator because it’s quick and simple. Shows 11.78 to 1 compression ratio, so you are probably right on your numbers.
Do you know how far down is the 1st ring from the piston crown? Only makes a tiny difference but I could not find it listed for the Icon pistons so I guessed .310” which may be off a little.
Did you verify the piston volume? At 4.5cc the notches must not be really deep.
