Don't get caught up trying to replicate the bean to closely, you got the right idea just need a little tweaking.
You have two options, convert to straight herbie or a modified version of beanaminal.
For Herbie:
Change the drain with the ball valve into a second back up just like the center drain.
Now just use the gate valve drain to adjust overflow water height just below the two back up drains. Be sure to test it with gate valve fully closed to insure the back ups can take full flow without tank overflowing, shouldn't be an issue but some make the back up pipe(s) too high.
For Beananimal
Change the drain with the ball valve to standpipe, durso, stockman or HGB, doesn't really matter but durso is more common.
The height of the standpipe needs to be lower than center back up but higher than primary gate valve drain.
You adjust gate valve so you get a little flow through the standpipe, with a small amount of water flow it should still be quiet but in theory not as quiet as pure herbie.
The advantage here is the system is more flexible, flow rate can change slightly and it should still be quiet, basically less need for periodic adjustment.
Here's a durso pic for ref.
