All - Everyone is different, but we ran into one Lady who owned the year before us, when we were picking up our new MH last year. She give us good insight into when to flush black tank, she does it every time she dumps. I asked her why, she said it keeps the sensors working better (although they maybe ultrasonic) and it gets rid of all the bad stuff in the tank. So here is how I do it.
Yes, there is a built in anti-syphon valve built in, I put another one (use Y connector, turn off supply to house when flushing).
I have a special hose dedicated to only flush out black tank, it's a coiled green hose. I hook up a anti-syphon valve and then a pressure regulator, then the flush hose. I do this while the black tank is draining.
Once tank is mostly empty, I turn on water supply and start the flushing process. I let this run for 90 seconds to 2 minutes, then close the black tank valve and with watch, time out 90 seconds - to 2 minutes. Once time elapsed I open up black tank drain valve and again let it run for 2 minutes or so. Then repeat the above three times. I have a clear pipe connector at the end of the sewer hose, and once it is running clear (or as clear as this can show) I am finished with the drain/flush process. I shut of flush water supply, it will drain out of the anti-syphon valve for a bit. Then I disconnect the hose from the supply, then take the AS and regulator off the hose bib. I let the green hose drain onto the ground for a bit. Then disconnect the hose from the coach, put the cap back on, and stow my stuff in the rear most drivers side compartment, small enough for not much of stuff, but it holds all this. I am anal, I attach the AS, WP regulator, and connect the male end into the AS so in effect I have a loop of sealed hose. The next time I don't have to look for anything it's all connected together.
As mentioned, I let the tank hit the 3/4 full mark, or the yellow light on the panel inside to light up. It takes a day or so, for anything to show on the tank gauges in our coach, this was true as well for the lady we ran into who advised us to flush every time we dump.
Have Fun!