Tableau: percentage of total for specific dimension / COUNTD IF statement

I have an employee dataset and I'm trying to display the proportion of BAME staff only as a KPI. My dataset has duplicate lines (staff who have more than 1 role) and so I have to use a distinct count of their staff IDs (i.e. cannot use measure values or SUM) as I want to measure ethnicity per person not per role. The only way I can get the % of BAME across the population is by having all other categories visible however I am looking for a way to display the % of BAME across total only. For example in a table calc BAME = 12%, Non-BAME=85% and Unknown=3% - I want to keep the 12% only to put on my dashboard.

I've not needed to do this in Tableau before and I'm quite new to the software. Is there a way to do this? I thought that perhaps I would need to do an IF statement similar to Excel but the below syntax returns an error.

IF [Ethnic Group]="BAME" THEN COUNTD([Staff ID]) / COUNTD([Staff ID]) END

Thank you so much in advance!