Recently, BT posted:

As long as the total of reimbursements are less than 50% of the cost
of the flight, it doesn't conflict with the FARs. OTOH, if someone
has a bug up their posterior and wants to hassle you, they don't
have to be right or interpret the FARs correctly.

Neil

As long as their total contribution is not more than their pro rata
share of the cost of the flight.
50% if there is only one person going with you.. but the example was
three others.

Reading of 61.113 (c) is pretty clear:

(c) A private pilot may not pay less than the pro rata share of the
operating expenses of a flight with passengers, provided the expenses
involve only fuel, oil, airport expenditures, or rental fees.

So, the 50% figure *is* the "pro rata share" that the private pilot must
pay *for the flight*. It doesn't matter how many contributors there are.

Neil