11,000 is definitely too low, as you can see on the calculator. Remember, these seem high, but they are not at all in comparison to the earth. You might want to check out this video on how much of the earth you can see at once. At 11,000 feet, you are probably seeing something like 1% of the entire earth. It's nothing. If you consider a globe, the Himalayas are only about a 2 mm bump on the surface of the earth. The scale of the earth is just enormous. Even satellites at 200 miles are only going to see 7% of the earth at once! Really, 200 miles is how high up you need to go to really undoubtedly see curvature.

This image below is the expected curvature from 200 miles! It's still not as much as you'd think, and probably what most people would expect from a high altitude balloon. Remember 200 miles is only the distance between Los Angeles and Las Vegas. It's a 4 hour drive. It is still tiny in comparison to the earth.

I don't know what independent scientists you are talking about, unless it's Globebusters. And they definitely showed a curve. Again, it's slight because it's only 100,000 feet, but it's there if you take a straight edge to it. I can show you at least 3 other independent rockets and high altitude balloons that show curvature.

Here's what happens when you measure the globebusters curvature. I personally am not even sure if these independent scientists are TRYING to find curvature, because sending a balloon up to 100,000 feet is not going to show much of a curve, so it's really a great way to confirm their bias, if that is their goal, while making it seem like they are trying to find the truth. The curvature expected at 100,000 feet is 0.6 degrees! Out of 360 degrees, that is virtually undectable by the human eye. It has to be measured.