There was a lot talk about new fuel flow system but very few quality answers given.
Since changes are quite a big from previous releases few tips that you should be aware of:
Fuel tanks were drained equaly from all tanks that are on same crossfeed level. (On simple craft it is within same stage)
You can easy change that behaviour trough mentioned "advanced tweakables" like it is mentioned.
Aasparagus staging can be easy adjusted with that info. Some quick craft example, as picture explains better than thousens of words:
Fuel tanks are on craft just for a sake of better explanation. FL-T200 is "root" tank stage, 2 x FL-T800 is second stage and outmost is FL-T400.
All decouplers have set crosfeed to enabled that gives new fuel flow feature even without fuel lines.
Without changing fuel priority, all engines were started and fuel is drained equaly from all tanks. That is not what we want. So, instead, I put "+1" in priority for FL-T800 and "+2" for FL-T400.
What happens in flight is that you got all power that you need from all engines for so much needed TWR. As soon as you got some speed, FL-T400 is drained first and you can eject them for even more TWR. On second stage all engines are drained from FL-T800 tanks. Once you drain them, you can eject them along with engines as you no longer need so much TWR for upper stage. You just need to be careful when you want to eject them.
All thing that you need to remember is to put "+1" on tanks that you want to drain faster than other tanks within same crosfeed level. I'm using term "crosfeed level" instead of stage because it seems more apropriate for this craft example.
There is still issues on some crafts, discovered trough KSP 1.2 beta testing phase. Fuel flow can be correct as descreibed, but visualisation of fuel flow can be wrong. That seems to be corrected for upcoming 1.2.x patch. More info about it is in this thread.
