Actually I might do this personnally

SET @boo = (CASE

WHEN @gross_pay < 35.17 THEN 0

WHEN @gross_pay < 151 THEN 40

WHEN @gross_pay < 201 THEN 50

WHEN @gross_pay < 251 THEN 60

WHEN @gross_pay < 301 THEN 70

WHEN @gross_pay < 351 THEN 80

ELSE 90

END)