i am providing a link to the IRS w-4 for 2020. step 4 line 4b is where the rental loss will
https://www.irs.gov/pub/irs-pdf/fw4.pdf
here's a suggestion if your return is rather simple
tax a piece of paper. write down your estimated 2020 salary
subtract from this the estimated rental loss and itemized or standard deduction
this is your taxable income.
compute your tax using the 2020 tax rate schedule
https://www.forbes.com/sites/kellyphillipserb/2019/11/06/irs-releases-2020-tax-rate-tables-standard-...
from this tax subtract $2000 for each qualifying child under 17
for other dependents (you are not a dependent) subtract $500
this should give you a decent estimate of what you need withheld
note that using your reduction in withholding to increase your 401K contribution has the effect odf reducing your taxable income.
what i propose is for very simple situations .
do note that if your income is too high, your rental real estate loss deduction might be limited.
