Another good example of something made a lot easier and less stressful by understanding a bit of basics. Adjusting the valves in concept is pretty simple. The precise location of the crank is not really critical. The idea is to rotate the lobe of the valve you are working on to be in a position where it allows the widest gap in the valve train, or in other words is as loose as it ever gets. Commonly this is convenient at top center on the compression stroke because both intake and exhaust valves are completely closed and you can check them both at the same time. In your case, you can verify that you have the cam in the right place just looking it it with the cover off and noting the rocker is on the heel, or base circle, or lowest part of the cam lobe. It does not actually have to be 180 degrees opposite of the high part of the lobe - most of the cam profile is the base circle, so technically you can use any of it.
The crank often does not like to stay put is any exact spot because forces from the valve train on other cylinders applies some rotational force, and/or the permanent magnets in the generator try to push it around. Don't worry too much about it. Look at the rockers on the valves you are working on while moving the crank back and forth at the marks.
You will see the crank moves, but it is not affecting the valves you are working on at its current position.
Keep in mind that valve lash is a bit counter intuitive. Too loose is way better than too tight. Loose, or too much valve lash may make a racket, but it takes quite a while to cause harm. Too tight, or not enough lash, and the valve will not close tightly enough to seal, allowing combustion gasses to leak through and burn the valve. That does not mean to not follow the specs, just to be aware that it is better to err on the loose side. If you think it is too noisy you can always re-check.