That's a good point Angus. Putting a dollar amount in the Constitution would prove unwieldy for adjustment in case of inflation.
The $300 would be for any given time or multiple times, but would best be built into the budget as in $1500 discretionary or miscellaneous, with a note that the slam masters could spend it in amounts up to $300 at a time so long as they both agree, and so long as they report it at the next Slam Family meeting. That should be enough to cover contingencies so long as the predictable stuff (including travel, registrations, venue rentals etc) is built into the budget.