I see what you are trying to accomplish with a indicator... But that experience comes with more seat time.. if you were doing some very fine grading for sand or something really important I can see it.
You want a simple version attach a small rod, yard stick anything that small enough to attach above the eye of the top link and point back towards the cylinder.. adjust it with the blade level to the point when it crosses plane with the cylinder that will be your level point. Seems more logical since you spend most of the time looking back... down towards the work the blade is doing.
I would suggest sitting in seat and looking back to get it to the point you desire before attaching by tack welds or other means.
A small rod, maybe a small clearance block of steel a nut or pipe to run it through and maybe a nut on top of you want it adjustable... You can drill a hole tap it or weld a smaller nut on top with a set bolt.
Just make sure it has clearance above the cylinder and won't make contact with it or hydro fittings when let in all the way.
View attachment 796105
Red clearance block if needed, blue pipe or nut, pink rod, green set point for adjustment
Small tack welds on that rod end is fine you're not getting anything hot enough to worry about.
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