how can I set up a dummy account to test?

I am trying to figure out how much I can afford to convert from Traditional IRA to Roth IRA for 2025 --- but by the time the software is ready, it is too late to run numbers  and make the conversion (conversions must be done by 31 December, right?) 

 

How/where can I find a simulation to help me figure out how much to convert without pushing me into the next tax bracket - and still have time to execute? And not take out all of my numbers from last year in my live account!