Why are you looking inside while turning? Do you have a light on the tool? You'll have a better horizontal control if you just watch the tool outside the vessel. That's a ton of height for hand hollowing. Before a lot of articulated or captured hollowing tools hit the market, the recommendation was to make a tool that looked like a harpoon to balance and control the cutting action. A Hunter #5 is a big cutter for hand hollowing. Even my articulated and captured rigs only use a 3/16" square piece of tool steel for the cutter.