How do i find this? i keep getting 31, but it should be 30

Your calculation for the cylinder volume is 31,792.50 cubic feet
I calculate it as 31,808.63 cubic feet
So, the 10,000 cubic feet mark would be located at
(10,000 / 31,808.63) * 45 feet =
0.3143800912 * 45 =
14.147104104 from the bottom
45 - 14.147104104  =
30.852895896  from the top
and that rounds to 31 feet
I get the same answer as you.