Do they make a 7/8" bulkhead? I've never seen any 7/8" pipe. I'm tempted to think you have a 1/2" bulkhead. Is it slip fit or female pipe threads? Remember the size of the bulkhead is actually determined by the pipe that slides/threads into it. And it's the hole inside the pip that's measured, not the outside diameter of the pipe.

In your first post in this thread you asked:
The correct response from me should have been:
Hell no! There is no pressure on a drain bulkhead! Only the weight of any water that is on top of it.(Weight and pressure are two totaly different things.)

My reply with the word "probably" was just a nice way of saying "No pressure....... Only weight."

BTW, there's nothing wrong with this calculator IMO. :)

Ok, so does the calculator assume there is no weight for the flow rates it spits out then?

I am not a physics expert, sorry if I used the wrong terms.

In my basic knowledge of physics, isn't one of Newton's first laws f = m * a, or force = mass times acceleration? Force is aptly named as N, for Newtons.
And water having mass, being pulled down by gravity at 9.81? m/s squared, will create force. That force when partially blocked at a bulkhead would create pressure, pressure expressed as Newtons per square inch? Maybe there is no pressure right at the bulkhead, just the surrounding glass?

I'm not sure, I just think you can put more through a bulkhead where there is more water sitting on top of the bulkhead than if there is none?