Something missing here is the distance standard.
There is either a time or distance airborne requirement. While pilots work in the time [from liftoff in my case], ATCO's, in the US at least, may use distance provided that the two a/c meet the distance standard when the second a/c lifts off.
In the 80's [and it would appear that not much has changed since, except in the UK] when we did a study to attempt to increase flow rates, the UK were the most consevative, the USA the least, and Aust was in the middle, where we've stayed.
There are elements of both judgment and luck in wake problems; sometimes it will still bite!!
G'day <img src="smile.gif" border="0">