I use a square to make the tang on the horn 90 degrees to the trailing edge. Its close enough to be undetectable. So if you don't feel it its not there so it must be perfect. This has always been a problem with engineers. That's one reason Burt Rutan was able to go into space from a junk yard. He didn't need a over engineered $5000.00 door handle he just used car parts to do the same job, that were close enough.
We're pulling away from your nifty build thread, so I'm going to try not to comment too much more on this.
You can go overboard in both directions on this. You can equate "good enough last time" with "perfect", then go on to build a failure, or you can equate "nothing is ever perfect" with "don't risk anything" and stand shivering on the brink of a big pool filled with money and never take the plunge.
Yes, when Burt Rutan is personally supervising the build you can use old car parts to build quality airplanes. But this doesn't work as well when you have a whole bunch of people working together on a project. Burt can do it because he's brilliant: he can look at a part and sum it up immediately. Burt is also not building airplanes in quantity, for great masses of idiot pilots. Lesser engineers, building for just anyone, have to analyze the bejesus out of any part, then insist that it be made exactly the same every time, with controls -- when you spread that expense over 100 airplanes, you end up with a $5000 door handle. I've seen the "it's perfect, just use it" attitude cause numerous projects fail, and I've seen as many products with exceedingly lame features leave a wake of irritated and disappointed customers behind, and I've even watched from afar as a client company collapse in the middle of a really fun project because "version control is too expensive". And, we're all seeing Boeing jet liners with batteries that burst into flame -- clearly they didn't spend enough money
there.
On the other hand, I've seen projects that I felt had strong potential never get launched because there was a Nervous Nellie at the helm, I've seen time and money spent flogging problems that were not there and
could have been solved with a hypothetical door handle from a junk yard, and I've had a client company collapse because the owner was unable to triage a bunch of problems in a new product, solve only the worst show-stoppers, and ship before he hit bankruptcy. You don't hear about those failures as often because they're not nearly as showy -- but it does happen, and if you keep your eyes open you'll see it happen.
If the surface flops down both when you hold the plane upright and inverted, then the amount of spring is negligible and you don't need to do any more work on the hinging and controls -- but the spring back is still there.