It isn't so much about just the size of the blank, but about how well the blank is balanced. If the blank is not well balanced, even 300 RPM is probably going to be too fast on a large blank. A well-balanced very large blank may be able to start significantly higher. The blank will tell you what is too fast -- when things start moving and getting scary. Sometimes even a perfectly round blank may be way out of balance too, because of moisture content, density, etc. Sometimes it is hard to tell until you try it. 345RPM is probably a good compromise in this case.