Latency level that is not noticeable to you while recording is a good amount. Around 12 ms is about where delay becomes noticeable. The alpha buffer setting is just a slider that I don't believe shows the exact size of the buffer. It has been a while since I owned one.
Many factors will affect the total latency of your system. As far as how low you can get it from Cubase, set the buffer (latency) slider as low as it will go without pops and clicks. F12 will open the VST Performance meter. If this hits red ever, then you need to increase the buffer size. Use as few vst plugins as possible when tracking as these will increase CPU usage and require higher buffer settings. After tracking, set the buffer high to allow you more time for CPU to process.