Compensation vs. Cost sharing?

Hey everyone.. Since I'm in Canada, and the rules are somewhat
different here in many regards, I'm pretty sure this will remain a
relevant question with a straightforward answer, none the less.

Where exactly is the fine line between being "Compensated" for flying
(contrary to a PPL) versus "Cost Sharing"...aka, my brother comes
along and offers to split the cost of a few hours rental.

Obviously, most people do this routinely, and even several of the
instructors at my flightschool have said that they gained a great
majority of hours themselves on the way to their instructor rating by
flying friends and familly around on shared-cost rentals.

However, Surely the "line" exists somewhere, and I'd sure like to
clarify it before I accidentally cross it and find myself in trouble.

Is it safe to assume (For example) that if I pay for exactly half the
flight plus 1cent (or any arbitray sum over 50%) that I am OK?

Is there an accepted percentage?

Any info appreciated..

Thx
Mark