1) For a certain time or frequency , it is difficult to know what component value to start with, which leads to a lot of trial and error attempts till you optimize your result.

2)Some chip parameters do affect your result.

SPICE software rely on Macros to solve problems like LM555 timers, Macros are ideal representation of the chip's equivalent circuit, but the manufacturer's equations are more accurate.

With Circuit Design, you enter what you need, the program will try every possible way to come up with the value of the missing parameters.
Then you can trim one value to the value you need , the program adjusts the results to fit the new situation.
Best of all , after you reach the final values that suites you, you may select the tolerances for resistors and capacitors, click optimize, then the program does 2 important things

1- It adjusts the values to the nearest practical value e.g. 512 ohms resistor is changed to 510 the value you can find in the real life.

2-Re calculate the circuit three more times,
Once with those new practical values,
Another with values + Tolerance Third with values - Tolerance.
the three sets of results are shown.
In the last stage if chip parameter interferes, it is taken into account too.
Now You get the best practical idea about what your circuit will behave.

* You can save the results.
* Copy and past them into any Windows(tm) application.
* Recall the saved circuit any time to review or re calculate . I hope you find this software useful