Can I make a "pre-forma" return on TT before the end of the tax year?

Yes if you use the Desktop program.  You could use the 2023 program by doing a test return or using the What-If Worksheet.   Or do a test return in the the 2024 Desktop program.  The Online version for 2024 has not opened  yet, not until December.   

You can use the TurboTax calculator TaxCaster to get an estimate of taxes owed or a tax refund.  The calculator is for tax year 2023 but will give a good estimate for 2024 - https://turbotax.intuit.com/tax-tools/calculators/taxcaster/

If you used the TurboTax desktop editions for tax year 2023 you can use the What-If worksheet in Forms mode for estimating 2024.  Click on Forms.  Click on Open Form at the top left of the Forms mode screen.  Type in What-If.

Thanks!

Thank you so much!

Thanks.

Can I do a what-if on the online version when it comes out in Dec?

Sorry no.  Access to the Forms and What-If worksheets are only in the Desktop installed program.  

Thanks.