vege until about 1/2 of the area is filled. they will fill in the rest after flip.
this is as much an educated guess as anythign else. there is variation in how much a plantn will grow after flip, but at minimum you should expect a doubling in size.
this gets much easier once you see it happen a few times.
take a pciture at flip...  then take a picture once vege growth stops after flip..  use it as a point of reference in future if needed.  you'll likely overgrow your first time  :P