Having a level on the tractor would be pretty much useless unless all of your yard has the exact same slope. Your calibration would change for each row. The location you've shown for the tilt gauge would likely not be sensitive enough to show you anything. If the end of the deck tilts up an inch, that area will probably move up less than 1/8".
How short are you trying to cut your grass? Most people seem to like to cut their grass really short, like 1 1/2", and that is too short if you have a rough yard and are expecting it not to scalp. At home in IL we have a pretty rough yard with some hills. I rarely have scalping issues with any decks because I mow the grass at 3". What size deck do you have? For rough yards, a 44" deck will do a better job than a 50" deck. Do you have 4 gauge wheels and the front roller? I added a rear roller to a 50" deck I have, and that helped a lot, too. What you'd typically do with the gauge wheels is adjust them so they're 1/2" above the ground at cutting height, but what you probably need to do is keep them on the ground at cutting height.
Is the float function of the rockshaft still free to move? Also, if the footrest support rod where the 'pickle fork' goes is worn, that'll add slop, too.
Those commercial mowers don't have a tilt adjustment because a) 99% of lawns would not need one and b) the operators would be losing money taking the time to make the adjustment. They are probably also using the largest deck they can, which is causing the same problems that I mentioned above.
You don't want to restrain the deck too much, or you'll make the problem worse instead of better. The deck needs to be able to tilt side to side to match the terrain- that's what the gauge wheels do- pick up the low side of the deck when it goes into a low spot.
All in all, I still think you may have some sort of problem with your deck and/or subframe that, when fixed, will solve most of your issues.
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