The practical approach
A family budget is not simply a single-person budget multiplied by the number of people. Housing size, childcare, school choices, transport and utilities behave differently.
Start with an appropriate home size, then add recurring household costs. Estimate the destination net income required to cover those costs plus a comparable discretionary buffer.
Only after calculating the target net amount should you gross it up through the destination tax system.
Related questions
Why not just convert the salary using an exchange rate?
Exchange rates convert currencies; they do not account for taxes, housing or household spending.
Should the result be treated as exact?
No. Relocation calculations are planning estimates. Personal tax circumstances and spending choices can materially change the outcome.