Your saddle isn't a little low, it is a lot low. With a sloping top tube like that you should have twice as much seatpost showing as you do. No point in having this forum if you can't say to someone that their saddle is 3" too low. Can you flat foot a stop without getting off the saddle? An earlier poster described the 'heels on the pedals' method, which requires the rider to know what a 'reasonable' bend in the knee looks and feels like. I will be the first to admit that left to my own devices, I am more likely to set my saddle too low than too high. There are more specific formulas involving carefully measuring your femur and multiplying by a constant (for some reason 1.09 is in my head) and that may be a valid starting point for experimentation.