May in accordance with the standard set forth in
section 206 of this title determine and prescribe what will be the rate or charge, or rates or charges, to be thereafter in such case observed as the maximum or minimum or both to be charged, and what regulation or practice is or will be just, reasonable, and nondiscriminatory to be thereafter followed:
Provided, That the Secretary shall prescribe the rate or charge, or rates or charges, on a percentage or per head basis at the election of the stockyard owner or market agency, or on any other basis elected by the stockyard owner or market agency unless the Secretary finds such other basis to be violative of
section 206 of this title; and