The number of berths at Nationals is fixed, 20 for the Mallory, 12 for the Baker and 16 each radial and full-rig for the Cressy.
At the end of the school year, the final allocations for the Cressy in the Fall and for the Mallory and Baker the following Spring are determined, using membership totals to develop proportions for each district. For example if a district has 70 member schools of a national total membership of 350, 70/350 = 0.2, which produces 4 berths of 20 for Mallory, 2.4 (rounds to 2) for Baker and 3.2(rounds to 3) for Cressy. To repeat : these are the final allocations for the Cressy in the Fall, and for the Mallory and Baker the following Spring.
Results are rounded and fit to the total number of berths in any one championship. All districts receive at least one berth in each of the three Nationals.
The shifts in allocations are the product of shifting proportions in member totals against the national total.