With a shim valve adjusted type engine, start by measuring the valve lash (the clearance between the valve stem and tappet). You'll need to remove the rocker box (valve covers) and rotate the engine to the correct position to do this.* There is a maximum and minimum clearance, I have a "Go/NoGo" feeler gauge that's just aces for this. The thickness of the gauge is the maximum clearance, then it's machined down to minimum clearance. So if the gauge will fit between the tappet and valve stem, it's not too tight, but it should stop at the ridge for maximum clearance, indicating it's not too loose. If this is what you have you're good to go. If the entire gauge slips through (too loose) or the thin part is too tight you'll need to adjust.
Now it becomes a measurement game. You'll need a set of thickness gauges in .01 mm increments, since you know the target clearance use the gauges to determine exactly how much change is necessary, measure the clearance, a good fit is required, then if the clearance is too tight you'll need to obtain a shim that is the difference between the clearance you just measured and the correct clearance.
Do whatever you need to do to swap the shim out, put it all back together and remeasure. Should be good.
Don't ask how I know...make sure it's sano. A spec of carbon or dirt in the bucket will throw the entire re-measurement off and you'll need to start over.
Observations - Air cooled engines (air heads, especially later ones) get a LOT of valve lash drift. These should be checked rigorously. 5000 mile intervals do it. Screw and nut adjusters. Easy. Oilheads are pretty low maintenance. After the engine settles in you only need to check them at 12,000 mike intervals. Heaxheads likewise, there is some drift that occurs. LQ engines, like my F-650, in 50,000 miles it's never required adjustment. From interval to interval it's always right on. I suspect
that our LQ boxers will be like this as well, but it's a new engine design, so a little "trepiddaciousness" at first will go a long way.
People have asked me (I don't know why they should) why valve lash drifts. Usually the valve lash gets tighter, this comes from the valve steam stretching or the valve head recessing into the valve seat. This causes less lash, then eventually none, then the valve remains slightly open eventually allowing ultra hot combustion gases to pass by burning the sh*t out of the valve and valve seat. Wah Wah... The lash becoming looser is less common, this comes usually from poor lubrication between the tappet and valve steam, or perhaps wear on the rocker heel and camshaft, also steaming from either poor lubrication or poor metallurgy. (See Chinese diesel engines)
Anyway, that's what I know...
* The Otto cycle, or 4 stroke engine rotates 720 degrees for every power stroke, so when you are looking for the right place to rotate the engine to set up the tappet adjustment marks realize there are two locations, find the right one where you have valve, the other one the valve will most likely be depressed down.
Here's a good video...