Local payroll expert needed to build a precise employment-cost calculator for Germany

We need a local payroll/HR compliance expert to build a precise employment-cost calculator for hiring a full-time employee in Germany. Not an average/estimate — we need exact calculation logic (gross → all employer taxes/contributions → employee deductions → total cost), including any regional variation within the country, residency/visa-dependent differences, and mandatory benefits. Deliverable: documented formulas + a worked example, not just a final number. You must also specify the exact list of input fields required to run this calculation accurately — which are mandatory, which are optional, and what happens to the result if an optional field is left blank. Also list the documents required to legally employ someone in Germany (mandatory vs. optional, and which trigger delays or issues if missing). Apply confirming your current hands-on payroll/compliance experience in Germany.