In this case, you need to set up a monthly budget for Sophia, accounting for her expected cash inflows and outflows. You need to calculate her after-tax monthly pay (cash inflow) using the after-tax pay calculator link given. This amount is the net pay she actually takes home, after all of the various taxes have been taken out (e.g., federal income tax, social security, etc.).
For the retirement savings analysis, note that the company will match 15% of Sophia's contribution, so however much Sophia can contribute, they will contribute 15% of that amount (where she can contribute up to $10,000/year if she can afford it).