Welcome to ReefBase
A 6x2x2.5? Nice tank.
Have you kept marines before?
Herbie method is similar to twin standpipe. You still need two holes. I'd use twin standpipe, there's lots of variation on exactly what constitutes "herbie" method, but twin standpipe is flood proof and the most reliable and silent tank to sump draining method there is.
You have two pipes, one taller than the other in the weir. The shorter one has a ball valve on it. Both go into the sump just beneath the surface. That's it. The water is restricted with the ball valve and so the pressure increases on it, and it finds a balance in the weir. If something blocks the ball valve or your return pump speeds up, it'll rise and drain down the unrestricted taller overflow, that'll make a noise, alerting you to the problem, but will take all the excess water no problem anyway, so it's faultless.
Do you have a picture of the sump layout? Which end of your tank is the weir?