"The TL431 has a reference voltage of about 2.5V, so the voltage divider for 5V is simply 2 resistors of the same value. Let's use 10k."
If it's 10k each, then 1 x r1/r2=2.5v. Shouldn't it be a 20k and 10k, so 1 x r1/r2=5v?
Vout=Vref(1+R1/R2)
for R1=R2 you get
Vout=Vref*2=5V
So R1=R2=10k ohm is correct like steve suggested
Rsup is the series resistor .
In order to calculate it you need to know the following info.
Maxiload=Max load current (Not "about")
Miniload=Min load current (Not "about")
MaxVsup (non regulated voltage)
Min Vsup (non regulated voltage)
once you know the above
IakMin =1ma (from data sheet) ;
RsupMin=MinVsup/(IakMin+Maxiload+5/20K)
IakMax= 100ma (from data sheet);
The maximum AK current in the circuit is
Iakmax=MaxVsup/RsupMin should be smaller than 100ma