Please be sure first and second calculations are not giving null.
Tableau wnt do arithmetic operations on Null.
Eg: Null-78= Null
## Replace Null with 0 may be gives you result.
IF [Target] = "HELOC Category 2017" THEN [Target group %] ELSE 0 END
By using Null functions replace the null value with 0 . i hope this will help you.