floobydust and Tim, thank you for your answers. Being a complicated subject, I'm going to sleep on your responses and see if I can come up with meaningful answers!
For now, I'd like to share my progress in determining K_fe, some problems I ran into, and maybe tips for others attempting to do the same. (Props to my dad, the math teacher, for helping.)
-----
1.) First mistake I made was reading the values off the core loss plot incorrectly. Notice on the x-axis (flux density), each tick is displayed at 100, 200, 300, etc. On the y-axis (core loss), the ticks are 10
1, 10
2, 10
3, etc. In between 10
2 and 10
3 are minor tick marks corresponding to 2*10
2, 3*10
2, 4*10
2, etc.
I however, initially read these as 20
2, 30
2. 40
2, etc. which is totally not the same thing! Don't do that!
-----
2.) I didn't really understand what the "core loss exponent" and "core loss coefficient" were, but now it is quite obvious. Core loss fits the power function model, that is y = a*x
b. In order to determine the optimum flux density that yields the lowest total power loss, that is Pcu + Pfe (copper + core loss), we need to know exactly what the core loss function looks like. In "Fundamentals of Power Electronics", K
fe corresponds to a, and beta corresponds to b. The optimum flux density is found where Ptot = Pcu + Pfe is minimized, that is the derivative = 0, i.e. at the bottom of a parabolic function.

-----
3.) While b, the exponent, can be found by determining the slope between two points on the core loss curve, I found this method to be rather inaccurate. Since core loss is on a log-log plot, small errors tend to be amplified, especially where the tick markets get compressed. Calculating by hand, using two points, I got lots of different exponent values, between 2 and 4. This error/variance means the calculated coefficient will also be off. tl;dr Doing this entirely by eye/hand does not seem very accurate.
A more accurate method is to select a number of points on the curve (4 to 6, maybe), and fit an equation to it, using Excel. In this case, I chose 4 points, and determined the exponent to be 2.83, and the coefficient to be 27.42W/cm
3*T
b. I'm much more comfortable with these values because they are quite similar to the "typical" values noted in the book. (Note, before I was getting coefficient values on the order of 500W/cm
3*T
b or more.)
