Thermistor calculations using B-Parameter equation
The T in the equation you reference is just the temperature that the thermistor is at. 

The assumption here is that you want to find the resistance R of the thermistor as a function of the temperature T.  

If you want to find T as a function of R, then you need the inverse of that function. 

Try an example: 

Say the thermistor measures 50.0K ohms, the temperature T can be found:

\$R_0\$ = 100,000

B = 3890

\$T_0\$ = 25°C = 298.15K 

\$r_\infty\$ = \$R_0e^{-\frac{B}{T_0}}\$ = 0.215628

T = \$ \frac {B}{ln(\frac {R} {r_\infty} )}\$ = 314.88 K or 41.73°C 

You can see that this agrees with the value shown in the table you linked: