We are turning wood here. They don't have uniform density. Moisture distrubution would affect the balance of the blank. The best tachometer is your gut feeling. If the lathe shakes, or you feel that it is spinning too fast, dial it down. Start slow, increase speed till I think it is too fast, then back off a little. After the blank gets more balanced, I will try to gradually increase the speed. I think that is how I can maximize the benefit of the variable speed feature of the lathe. Only the confience can tell me when I can increase the speed, not the rpm read out.