I take the flagstick to be 5 feet high and I use that to calculate distance.
The critical off-set measurements are those of your approaches. In general, the accepted value for flag-stick height is 7 ft (~2 meters for our int'l players). Since you're working with the green itself or near-green, the game gives you a nice measuring device and terrain analysis tool: from off the green select either the forward or reverse green zoom view (reverse not available on all holes, alas, as that's my favorite for making windage adjustments. Now select pitch, chip, or flop shot type, and with aiming point on the green, left click on the aiming point. You'll see this type of display:
The square terrain area shown is a box 10 yards on a side (14 yards diagonal) centered on the aiming point. In-game, that terrain box comes complete with moving dots for break, etc.; if the display is too small, right-click on the aimng point and Zoom In to see the details. The distance from the aiming point to the mid-point of any side of the square is 5 yards (15 ft), a bit more than 2 flagsticks (~14 ft).
While left-clicking, drag the aiming point around the green-side area - as long as some part of that terrain square touches the green/fringe, you can see the entire square's terrain, including 10 - 14 yards off the green proper. Using this technique you can map the entire green for localized slope.