The reasonableness standard appears in a number of places in the Income Tax Act. You're absolutely correct that ultimately it would be the court, and going up to the Supreme Court, that decides questions of how statutes ought to be properly interpreted.
This reasonableness test doesn't look to an abstract reasonable level of income based upon your investment. Rather, it looks to a reasonable allocation of your income between you and your relatives vis-à-vis a business. This is one of the refinements made from the version that was released in July. It's not asking, if you invested $100 and you put in 50 hours of labour, what's the absolute value number of what you should be able to get. Rather, it looks to the relative contributions of both the individual receiving the income and any of the relatives who are involved in the business to see if it can reasonably be considered as appropriate. Again, it's not a specific number. It's more that it is in a reasonable range.
Lastly, it builds upon an existing reasonable allocation rule that applies currently, and has for some time in the context of partnerships, in subsection 103(1.1) of the act, where they look to whether or not an allocation of income or other amounts from a partnership are reasonable, having regard to the contributions of labour and capital by each partner to the partnership.
That's something where there's long-standing Canada Revenue Agency guidance. There has been some case law on it. It has been used for quite some time and I think people have gotten comfortable with it. In this context this builds on some of those same ideas. The Canada Revenue Agency has provided guidance. You're absolutely correct that if a taxpayer doesn't agree with the Canada Revenue Agency's assessing position, it can be appealed to the courts for an ultimate determination.