I have the following:
set @SomeVariable = @AnotherVariable/isnull(@VariableEqualToZero,1) - 1
If @VariableEqualToZero is null it substitutes t
SET @SomeVariable = @AnotherVariable / COALESCE( CASE WHEN @VariableEqualToZero = 0 THEN 1 ELSE @VariableEqualToZero END, 1) - 1