Les, it wasn't a direct reply to your last post, just a general one to some of the others and to other advice I have heard which doesn't really pin down what can be considered a safe approach to speed selection on the lathe.
Steve,
Yes the formula simply gives the rpm to achieve a given peripheral speed of the wood past the tool for a given diameter and as I said at the beginning of that explanation, the overriding factor is balance and safety and it is just a rough guide for a generic piece of wood but one that gives a safe upper limit in normal circumstances.
The formula is the same for every lathe. The bit that differs is how a particular lathe can cope with vibration and imbalance.
What I don't think is good advice is to suggest to increase the speed until there is vibration because for a well balanced piece of wood, there is a good chance you can considerably exceed a safe working speed before there are any noticeable vibrations so in my opinion, a method of determining a safe upper limit is needed which isn't reliant on someone's limited experience or perception.
Digressing slightly - balance is a complex subject in its own right. I'm not an expert on that but from what I have learnt, you can't always treat something as a simple mass. The actual balance at different points along any object might vary, that's why when they balance car tyres they sometimes end up with one weight on the outside of the wheel and one on the inside. It is possible to balance a piece of wood with counterweights, I have done it in the past with eccentric turning but for most occasions, we just have to remember to limit speed to below the point where the vibrations caused are more than the machine and mounting points for the wood can safely cope with and a decent cut can be achieved.