How to calculate correct value of capacitor C2?
So you have tried values, found one which works, and are done? 

The voltage between R1 and C2 (EN pin) follows the law of the RC time constant. So initally C2 is discharged and EN = 0v. \$\tau\$ = 200k*100p = 20µS. So 20µS after power is applied, V_EN will be ≈ 63.2 percent of Vcc or 2.09v. (Loading from VREF2, EN, and T1 may alter this.)