Notional goodwill & impairment loss

Dear tutor,
With NCI at proportion of net assets

Less say we have imp loss of $30,000
Then share of parent is 80% & nci is 20% & goodwill of $48,000 (notion g/w is $60,000)
Then we will have actual impairment loss attributable to parent only of $24,000 ($30,000 x 80%).
In another scenario, i take out only notion goodwill attributable to nci of 12,000 (48,000×20%/80%)
Then we will have impairment loss of 18,000 (30,000-12,000)

It seems unclear for me about these imp losses

Can u explain for me the difference between the remainder of impairment loss after stripped out grossed up goodwill (notional attributable to NCI)
And the actual impairment loss (parent % x total impairment loss)
Which figures should we use to write down into g/w and post acq profit? Which scenarios should we use?

Thank u in advance