A district that receives a petition under § 49.503 shall compute the proportionate amount of water rights to the Rio Grande. The proportionate amount of water rights is equal to the amount of irrigable acres of land in the subdivision multiplied by the lesser of:
(1) 1.25 acre-feet per irrigable acre; or
(2) the sum of all irrigation water rights owned by the district on September 1, 2007, as if the water rights had been converted to municipal use under applicable commission rules, divided by the total amount of irrigable acres of land in the district on September 1, 2007.