Cuboid in Cylindrical Water Tank

A cuboid has dimensions $7$ by $8$ by $x$ where $x > 8$. A water tank is a cylinder with base area $a$. The cuboid stays at the bottom when water is added.

When the cuboid has the $8$ by $x$ face down, a volume $v$ of water is added to the tank to a depth of $7$, level with the top of the cuboid.

When the cuboid has the $7$ by $x$ face down, the water level drops by $1$.

When the cuboid has the $7$ by $8$ face down, the water level again drops by $1$.

Find how much the water level drops when the cuboid is removed.