The number is not known ahead of time.
It depends on the ending payrolls for all teams and the end of the season. The total payroll divided by the number of teams in the league determines the average. The luxury tax is applied to those teams that had a payroll over the average. Since the final payroll for each team is variable as players are signed and traded, you won't know what it is until the season ends.
The final number will appear on the Accounting Tab in the Front Office Tab. The line item is called "Revenue Sharing".
The only planning you can do to know if it applies will be that your payroll is very high all season long. Odds are greater the tax will apply to your team.
See the manual for more info:
http://manuals.ootpdevelopments.com/...age=financials
Search for Luxury Tax on the page. It is near the bottom.