Determining current draw and Ah requirements from a multi-component system
If component one draws 300ma in a steady state, and component two draws 150ma in a steady state, and component three draws 25ma in a steady state, that is a total nominal draw of 475ma while the system is in a steady state (user defined of course).


Correct. The current the power supply delivers is the sum of the load currents.

300 mA + 150 mA + 25 mA = 475 mA.


  The question is, is this a rough, first pass way to determine total current draw from multiple sources being run by a single power source, 


Even better: it's as good as the current measurements are. That's the way the professionals do it.


  2) determine battery capacity for a desired run time? aka, 4.75Ah capacity for 10 hours in this scenario.


Correct. Current x time (in hours) = Ah.

Just remember that for most battery applications you should allow for more than it says on the tin. Batteries degrade over time and generally last longer if you don't discharge them completely - particularly in multi-battery packs as the first to get depleted then has reverse current driven through it by the others. As a result you might go for 1.5 x Ah estimated.