Herbie's original design was used in a corner flow tank, IIRC, meaning you could have a large vertical distance between the siphon inlet and the emergency pipe. The additional head pressure generated by a the higher water level would help to auto regulate the system. Somewhat. Like you said, they tend no to be great at auto regulating and require frequent tinkering unless you decide to go the 'wet' herbie route, which is what most people end up doing.
This is why I still like in-tank overflows with holes in the bottom. Yes, you lose some tank space, but you gain some flexibility since you can get a siphon started easier and don't have to be quite as precise with the adjustments, given the larger vertical space available. Plus, since that's still the way most "reef ready" tanks still come, it's nice to have overflow plumbing methods that work well with that design, versus methods that require a different approach. That said, the last herbie I ran was on an overflow through the back with very little room. I was able to dial the flow on the siphon well enough that it went a few years with zero "tinkering." IME, a lot of reefer "tinkering" is self-inflicted. Once things are dialed in, there shouldn't need to be adjustments.
I do think, ideally, the 'emergency' would be larger than the siphon; but most mass market tanks don't offer this.
If you have two holes of identical size, and use one for a siphon with a gate valve that's partially closed in order to regulate flow, and have the other as a straight shot with no restrictions as the emergency overflow, then essentially your emergency is "larger" than the siphon, since it doesn't have a partially closed gate valve on it - even though the bulkheads are the same size. As long as that emergency drain can operate under siphon without sucking air, you've got excess capacity in your emergency compared to the primary siphon. That's why, IMHO, it's OK to run a "wet" herbie, since the small trickle in the emergency is likely not going to cause a big enough loss of capacity that the full flow can't be handled.
The good news with all of this is that it's really easy to learn in a hands-on manner. IMHO, the biggest mistake you can make is to take a formulaic approach and build something you don't understand. There are LOTS of ways to run a reef tank, drain plumbing included. Building something you don't understand is risky. Instead, I think it's better to get your hands wet (literally). If you're trying to build something that can handle an emergency, simulate an emergency and see what happens. Block off your siphon with a rag and see what happens. Unplug the pump, then plug it back in 10 seconds later before the tank has drained down. Then do it again, but let the tank drain down fully this time. Then unplug the pump and plug it back in with the rag blocking the siphon, and see if the emergency can pull a full siphon from scratch. Think up any possible scenario, then test it, and adjust if you feel you don't like the result.
