AUSMHLY, Here's the page from the Bentley Manual for the Big Healey. I believe that the centrifigal advance starts to come into play just at or a tecsh beyond 600 rpms. So if your setting the timing at whatever at an rpm above 600 you are allowing the centrifigal to do some of the base/initial set point. If then you were to slow the rpms down to 600 you would find that your base/initial is alittle lower that what you set it at. Unless you are racing that isn't going to affect you to much. As a matter of fact, at the lower base setting at 600 rpms it will go a long way to keep your engine from pinging or running on after shut-off. But I generally have my timing anywhere from 10 to 15 degrees BTDC at around 800 rpms. Here's the page: