According to the text books, focal length is measured from the mid point inside the lens but to be practical an external surface has to be chosen. As far as laser heads are concerned once the best point of focus has been obtained then measure from any convenient metal surface.
From my physics days (long ago) the best power from a plano-convex lens was from the shortest depth of field which would be curved surface towards laser and flat surface towards work.
With the curved surface towards the work the usable depth of field is greater but the available cutting power is less.
On my system it is curved side up but looking at the cheap Chinese machines they are usually fitted curved side down.
Nothing is ever conclusive.