BleachCalc has a known bug when it comes to Borates. 1 box, which is 76 ounces weight, of 20 Mule Team Borax raises the Borates level by 6.45 ppm in 10,000 gallons. You can use The Pool Calculator for a more accurate calculation. For your 30,000 gallon pool you would need 23-1/4 boxes to get to 50 ppm.

The Borates measurement is technically in "ppm Boron" which is probably why BleachCalc is incorrect as it is using a different molecular weight.