[Reiver]

I have a spring load on both sides but always adjusted in the 1/2 plane and it was essential to keep the shifter stalk 'centered' as it will fall to one side or the other.
It has been a long time since I've done this so I am trying to remember what I do with this.
I first 'feel' the engagement of depth....between 1/2 or 3/4...you should have the same 'throw' for and aft...if not you will need to adjust the forward/rearward position of the coupler.
Then I check the lateral by going into 1st gear...holding the shifter in the middle of it's 'play' left right and check going laterally to 3 or 5th....if it will not go to 5th then some lateral adjustment is in order (mark your for /aft position as that will be easy to lose when you loosen the coupler).
You are searching for the sweet spot and it takes some attempts.
I also would drive a bit when it would shift into all gears...if I get too much 'noise' from the box then I adjust for and aft a bit.....it is usually a depth issue then.

As to your question...I put it into 1st gear and 'center' the shifter in whatever 'play' there is in that gear....does that make sense? Visualize what is taking place. The shaft into the tranny only has so much movement. When in first gear center the shifter when tightening the coupler...then try 5th/r...if it won't go into those gears then loosen the coupler and give it a bit more movement/engagement in the direction of 5th/r. Keep doing this until you can engage all.
If your depth...for and aft, is off you will often hear too much mechanical noise from the box as the gears are not totally meshed....then you adjust for or aft until you have a similar sound form 2 and 3rd....

Welcome to the intricacies of the 915....you'll get it...the more you try the more you get the feel of what is taking place.

At one time early on I thought this kind of thing a pita....it is a bit but it is also 'personality' and when you figure your 911 out you get this connectivity.
I've had my SC for 10 years now, rebuilt numerous systems myself and 'know' the car like no other...the damn thing talks to me when I drive it...swear.