Sometime a better gage than your thumb is to sit on the bike with your full weight and see how much the tires depress, or how large the contact patch becomes. When riding, the danger is that a sharp jab on the tire, like hitting a pothole, will compress the tire all the way to the rim and result in a pinch flat. I assume the tire may see several times your weight in a dynamic condition like that, so you're balancing that possiblity with an adequate contact patch for traction.