Welcome to the forum!
There are glass thickness calculators on line but the ones I am familiar with are for aquariums. There could be some for your pond application. If you strike out, you might contact a public aquarium and ask them to provide the information for the manufacturer who built their smaller display tanks that are often called focus tanks. The Mississippi River Museum in Dubuque, Iowa has sensational display tanks with glass inserts like you describe. Specifically, the display tank in the main area that contains large fish and turtles is exactly as you describe. Bass Pro and Cabella's are another possibility. The Wonders of Wildlife Museum in Springfield, Missouri is another excellent resource. They have many tanks like you desire. Sorry to 'spam' you with an answer but your project is most fascinating and I am excited to see how it turns out. Again, welcome to the forum.
Stephen