The actual value of inv_erf(1-10^-20)=6.60158062235514256151639163241871..., I believe.  I used my own solver, a cubic version of Newton iteration called a Schroeder series, but with an extra step that  helps convergence go faster.  I did 9 iteration steps.  Actually, I used Digits:=5000. This tok less than 5 sec on my high-frequency Apple MacBook Air

However, it took MAPLE  a time of .42 sec to evaluate erf(6.60158062235), comiing very close to the desired value     

                  *   *   *        erf(x)=1-10^-20       *   *   *

 The actual value found was

                   *   *   *   erf(x)=0.999999999999999999990000000000648...  (from 5,000 dits)   *   *   *

and the last digit rounded down to '5' was very close.   But even then, with Digits:=5000,.MAPLE was not able to calculate erf(x) correctely for the longer value of x above stated to 32 decimals.  It did seem to help when I used fewer decismals, like the 11-decimal argument mentioned.