When felting while i always put welted verge on i never put welted drips on and have not done so for well over 15 years.The reason i dont is because as you will know many flat roofs are just too flat with little or no fall.When putting a welted drip on you off course end up with some sort of ridge along the front once the cap sheets are fixed,more if using 4mil underlay as i do.This often causes ponding along the front.If im wanting to put 2" drips i cut 1 3/4" strips of mineral and nail these into the drip lath mineral side facing the lath ie so the bitumin side is showing.I then set my sheet of felt allowing a 2" drop and then stick the sheet and drip in one.Now although its a single drip it does have the strength of a welted drip and is also secured with nails like a welted drip but removes the ridge and in many cases the ponding.