It's a fairly standard dual adjustment design.
1. The orifice generally sets the overall relationships in the air barrel and will place the poppet in the proper position relative to the tabs on the the lever to set the lever height.
2. The spring pressure is then adjusted with the screw to adjust cracking effort.
With regard to number 1, the lever does not have to be in contact with the dipahragm and that may not be ideal due to case geometry fault issues. But you do want to ensure that you do not have excessive play as that will reduce the working range of the valve and reduce the flow rate. You want to ensure when you are done that you can a) fully purge the reg and b) do not have to depress the purge more than about 1/16" to 1/8" to initiate air flow. Note also that very small adjustments of the screw will have a large impact on cracking effort, so the finaly tweaking may be 1/12th of a turn or so.
The seat will also develop a seating groove over a period of a few weeks on the shelf or over the period of a few dives. That may result in a slight freeflow that may again require a slight adjustment of the orifice or spring pressue.
Finally, there is some potential interaction between the two, especially if you make large adjustments to one or the other, so you may have to loop back around and fine tune both adjustments.
With regard to number 2, in terms of initial inhalation /cracking effort, anything less than about 1.0 inches of water may result in a slight freeflow wne you are in a face down postion. This is aggravated slightly if the lever is adjusted so that it is in contact with the diaphragm rather than having a very slight amount of slack.
