I'm primarily a hobbyist with occasional production runs. Instead of just using experimentally determined speeds, feeds, and depths of cut that sound and look "right", I'd like to determine these parameters using science instead of intuition. I know there are many variables (material, machine horsepower, cutting tool style and composition, machine rigidity, ...), and I've read articles about productivity "sweet spots" that must be empirically determined, but I haven't found one really good source of information that puts these all together in an easy-to-digest format that suggests a good starting point, and then essentially a flowchart that takes one through the steps to find a sweet spot.

I've found many tables with a wide enough range of data that it's almost unclear where to begin. Textbook data seems to presume a very weak machine, while commercial data seems to presume machines with infinite power. My own machine (2hp) is somewhere in between.

It seems that I select the speed first (in surface feet per minute), based on the material and cutting tool composition, so as not to wear out the tool too fast. Then I determine feed (inches per revolution), based on the tool nose radius for the desired surface finish. Then I determine depth of cut (inches) based on machine power.

Is this all there is too this? Am I making this more mysterious than it needs to be?

That's it, you've got it! Great, now you've exposed our secrets, how are we gonna be able to charge like bandits henceforth? :D

In real life, if you want to max out your lathe, you might make good use of an amperage meter, because otherwise, the proper load is a whole lot of guesswork.

The rigidity of the setup will also act as an unexpected factor to wreck the theoretical figures, as will the actual chip breaking abilities of a particular type of insert and material. Varying feedrates and depth of cut to force proper chip control is done quite often.

Like Hu says, you look at the chart, you look at the machine and setup, you think about the things you have turned in the last 20 years (and how that has all worked out) then you pick numbers while holding your tongue in the correct position.
Once you start cutting you make constant adjustments untill you are satisfied.
Clear as mud eh

Well, maybe I'm not as far off as I thought, but what is the best reference that provides a "chart" to determine the starting point before the trial & error tuning takes place ... something that cleanly and concisely lays out a starting speed, feed, and depth for combinations of workpiece and tool materials?

As I said, it is clear as mud.
There are too many variables.
The clearest explanation you have already posted yourself the first time.