% with static denominator

Try VALUES instead of KEEPFILTER

Referral % = 
VAR CurrentCount = DISTINCTCOUNT(Data[referral number])
VAR BaseCount =
    CALCULATE(
        DISTINCTCOUNT(Data[referral number]),
        ALL(Data),
        VALUES(Data[Date])
    )
RETURN
DIVIDE(CurrentCount, BaseCount)

Hi micang ,
You can try this approach after using the measure shared by Sahir_Maharaj 
 Create a Filtered measure-
 FilteredReferralCount =
DISTINCTCOUNT('Data'[Referral Number])
This will respect all the slicers.
Then create the final measure-

ReferralSharePct =
DIVIDE([FilteredReferralCount], [BaseReferralCount], 0)
Hope this helps!
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Thank You