Grossup on imputed earnings setup

I am still looking for help on Grossup elements setup on imputed earnings.

Looks like Oracle only allows grossup setup to be only on supplemental earnings, and the supplemental earnings in my current environment are using flat tax rate calculation.

But my payroll team wants to have gross up setup on earnings/imputed earnings, so that grossup takes into consideration employee level tax for calculating fed, med and SS, instead of the flat rate.

Has anyone come across this requirement. Any setup steps or documents will greatly help.

this is how they would like this work, for example:

*Housing Allowance amount per agreement* ==