I flop exclusively between 7-25 yds (from fairway up to 40% rough).
I also did it from sand exclusively until I read a tip on sand play from Gary Pinhunter's site.
I always use reverse view to move marker. a 10 yard flop, move the marker to the right slightly less than a full square. for 20yd flop, 1.5 squares to the right. Always remove the pin and always full BS.
a 15yd flop with your 20yd Cleveland wedge, is going to take about 85% power from the fairway, almost full power from 30% lie.
I would suggest taking a sleeve of balls and play a practice round (using the above data as a starting point), putting yourself in the rough and fairway at different distances and charting your shots or coming up with a formula/head math to use.