Proper girdle size relates to intent. The .3mm specified in the USFG gives an additional challenge to competitors to cut and measure their stones to this smaller, more critical dimension but it also allows judges to better see whether the girdle line(s) are level/parallel. With a thinner girdle, unevenness sticks out easier.
However, this thinner girdle (depending on cut to a degree and the material of the stone) is too delicate for most jeweler’s tastes because there is a real danger of them cracking a stone while setting it. I don’t have much experience setting stones so when a coworker asked me for some help with an anniversary gift of a ruby to be set in a ring he had already purchased, I acted as an intermediary between a local jeweler I have done business with and my friend. The loose stone was purchased on Jewelry TV and was an off size, as was the ring he brought in. My guy can work around the off-size issues but the ruby had a large ugly pit in it, very near the edge of the stone which had a fairly thin girdle as it so happened and no pavilion (there was no need with this opaque material).
When inspecting the stone I saw the pit at a distance with my naked eye so you can imagine its size. I brought it to the shop and my guy was a little intimidated to try setting it as-is. He initially wanted to relocate prongs away from this vulnerable area but after sitting with it for several minutes at the bench, decided to go for it and I was really thankful the stone did not crack. I don’t know what my friend paid for the stone. He seemed proud of his purchase (as if he had spent a fortune) but I looked at a cloudy, poorly cut ruby and shook my head at the thought that he wished to proudly present this to his wife of 50 years whom he adored.
For my taste, I would cut a girdle a little thinner than what you show in your photograph, perhaps about 80% of what you have. There is a sort of feng shui consideration (rules of proportion) that you need to employ. If the girdle is thick enough to be visually distracting from the rest of the cut, it is too thick. Basically when you look at the stone you should not notice that the girdle is there, but it needs to be thick enough to be sturdy and of course there is the idea that a thicker girdle allows for slightly more carat weight and a little deeper color saturation.