Seemed to correct himself as just basic physics ....

Message:

The external pressure would be constant in the water. This is to compensate for the motion of the water which would affect the pressure.

This is hard to do to compensate for the water being lost. (Our brain would assume that water was not being added and that would change the answer.)

Therefore the only variation is the height of the head of the fluid (water?).

As you go below the mid-point, the time in free-fall reduces.
As you go above the mid-point, the velocity reduces.

Simple ballistics to calculate the distance it would traverse. I guess it's technically calculus but we know d = 1/2 a t^2.and d = v * t.

https://cdn.ymaws.com/www.eps.org/resource/collection/016775D4-8888-474D-887F-3E33AEA5E6D0/EPSPED_MUSE_bot_holes.pdf

Moving the surface at the bottom changes the distance of course.

Since you can't easily get the liquid at the top to maintain, it's damn near impossible to perform the test (and that's why the professor didn't do it either in the video).

Yours,