You can’t detect if you are on the firing stroke by looking at the flywheel. The flywheel makes two complete turns to accomplish the four cycles, so the flywheel mark will also be aligned as the exhaust cycle has ended and the intake cycle is beginning at the end of the first rotation. Your camshaft gear(s) markings need to be aligned simultaneously along with the flywheel markings. Most often this positioning is marked at the top dead center position on one cylinder recreational engines but it’s certainly not a rule. In this position your cam lobes should be sitting in an upward facing V-orientation. To measure and adjust the valve lash, I would suggest rotating each cam lobe so you are measuring the clearance on each lifter on the opposite side from the lobe. On an overhead valve engine you can do this whole adjustment with the head on the bench as an option if you like, unlike a pushrod engine. You do have to snug down the cam bearing caps to hold the camshaft in its proper position.