It definitely depends on the club and distance too. A green that is +15' but is only 90y away? Wedge basically moves more on verticals, so I'd only add 1-2y, however if the green is 190y away, I'd add 5y. Downhill lies will play shorter for lower irons because they don't fly as high so come in at a less steep plane. For example, a 6 Iron at landing will be dropping at a more steep angle than a 3 iron, thus will require more flight distance AND will roll less at landing.