Using a framing square measure the depth of your door jamb by sliding the square under the newly cut gap until the tool hits the door frame or stud.
Laminate under door trim.
I made a quick video on how to undercut door jambs for laminate flooring it is a diy job most people could undertake themselves if the have some basic tools.
The 1 2 inch gap is for the doorway only.
Measure on the inner jamb as well as both sides of the door where the trim is located.
When installing laminate flooring obstacles such as doors present a problem because you have to make cuts for a correct fit.
Install the last row of laminate flooring stopping 1 2 inch from the bottom edge of the patio doors.
If there s a gap under your door casing and also a gap between two rooms under a door where two different installations of laminate flooring join you can install t molding to hide both gaps.